## Maharashtra Board 12th Maths Solutions Chapter 8 Binomial Distribution Miscellaneous Exercise 8

Balbharti 12th Maharashtra State Board Maths Solutions Book Pdf Chapter 8 Binomial Distribution Miscellaneous Exercise 8 Questions and Answers.

## Maharashtra State Board 12th Maths Solutions Chapter 8 Binomial Distribution Miscellaneous Exercise 8

(I) Choose the correct option from the given alternatives:

Question 1.
The mean and the variance of a binomial distribution are 4 and 2 respectively. Then the probability of 2 successes is
(a) √50
(b) 5
(c) 25
(d) 10
(b) 5

Question 2.
The mean and the variance of a binomial distribution are 4 and 2 respectively. Then the probablity of 2 successes is
(a) $$\frac{128}{256}$$
(b) $$\frac{219}{256}$$
(c) $$\frac{37}{256}$$
(d) $$\frac{28}{256}$$
(d) $$\frac{28}{256}$$

Question 3.
For a binomial distribution, n = 5. If P(X = 4) = P(X = 3) then p = ___________
(a) $$\frac{1}{3}$$
(b) $$\frac{3}{4}$$
(c) 1
(d) $$\frac{2}{3}$$
(d) $$\frac{2}{3}$$

Question 4.
In a binomial distribution, n = 4. If 2 P(X = 3) = 3 P(X = 2) then p = ___________
(a) $$\frac{4}{13}$$
(b) $$\frac{5}{13}$$
(c) $$\frac{9}{13}$$
(d) $$\frac{6}{13}$$
(c) $$\frac{9}{13}$$

Question 5.
If X ~ B (4, p) and P (X = 0) = $$\frac{16}{81}$$, then P (X = 4) = ___________
(a) $$\frac{1}{16}$$
(b) $$\frac{1}{81}$$
(c) $$\frac{1}{27}$$
(d) $$\frac{1}{8}$$
(b) $$\frac{1}{81}$$

Question 6.
The probability of a shooter hitting a target is $$\frac{3}{4}$$. How many minimum numbers of times must he fie so that the probability of hitting the target at least once is more than 0·99?
(a) 2
(b) 3
(c) 4
(d) 5
(c) 4

Question 7.
If the mean and variance of a binomial distribution are 18 and 12 respectively, then n = ___________
(a) 36
(b) 54
(c) 18
(d) 27
(b) 54

(II) Solve the following:

Question 1.
Let X ~ B(10, 0.2). Find
(i) P(X = 1)
(ii) P(X ≥ 1)
(iii) P(X ≤ 8).
Solution:
X ~ B(10, 0.2)
∴ n = 10, p = 0.2
∴ q = 1 – p = 1 – 0.2 = 0.8
The p,m.f. of X is given by

Question 2.
Let X ~ B(n, p).
(i) If n = 10, E(X) = 5, find p and Var(X).
(ii) If E(X) = 5 and Var(X) = 2.5, find n and p.
Solution:
X ~ B(n, p)
(i) Given: n = 10 and E(X) = 5
But E(X) = np
∴ np = 5.
∴ 10p = 5
∴ p = $$\frac{1}{2}$$
∴ q = 1 – p = 1 – $$\frac{1}{2}$$ = $$\frac{1}{2}$$
Var(X) = npq = 10($$\frac{1}{2}$$)($$\frac{1}{2}$$) = 2.5.
Hence, p = $$\frac{1}{2}$$ and Var(X) = 2.5

(ii) Given: E(X) = 5 and Var(X) = 2.5
∴ np = 5 and npq = 2.5
∴ $$\frac{n p q}{n p}=\frac{2.5}{5}$$
∴ q = 0.5 = $$\frac{5}{10}=\frac{1}{2}$$
∴ p = 1 – q = 1 – $$\frac{1}{2}$$ = $$\frac{1}{2}$$
Substituting p = $$\frac{1}{2}$$ in np = 5, we get
n($$\frac{1}{2}$$) = 5
∴ n = 10
Hence, n = 10 and p = $$\frac{1}{2}$$

Question 3.
If a fair coin is tossed 10 times and the probability that it shows heads (i) 5 times (ii) in the first four tosses and tail in the last six tosses.
Solution:
Let X = number of heads.
p = probability that coin tossed shows a head
∴ p = $$\frac{1}{2}$$
q = 1 – p = 1 – $$\frac{1}{2}$$ = $$\frac{1}{2}$$
Given: n = 10
∴ X ~ B(10, $$\frac{1}{2}$$)
The p.m.f. of X is given by
P(X = x) = $${ }^{n} C_{x} P^{x} q^{n-x}$$

(i) P(coin shows heads 5 times) = P[X = 5]

Hence, the probability that can shows heads exactly 5 times = $$\frac{63}{256}$$

(ii) P(getting heads in first four tosses and tails in last six tosses) = P(X = 4)

Hence, the probability that getting heads in first four tosses and tails in last six tosses = $$\frac{105}{512}$$.

Question 4.
The probability that a bomb will hit a target is 0.8. Find the probability that out of 10 bombs dropped, exactly 2 will miss the target.
Solution:
Let X = the number of bombs hitting the target.
p = probability that bomb will hit the target
∴ p = 0.8 = $$\frac{8}{10}=\frac{4}{5}$$
∴ q = 1 – p = 1 – $$\frac{4}{5}$$ = $$\frac{1}{5}$$
Given: n = 10
∴ X ~ B(10, $$\frac{4}{5}$$)
The p.m.f. of X is given as:
P[X = x] = $${ }^{n} \mathrm{C}_{x} p^{x} q^{n-x}$$
i.e.p(x) = $${ }^{10} \mathrm{C}_{x}\left(\frac{4}{5}\right)^{x}\left(\frac{1}{5}\right)^{10-x}$$
P(exactly 2 bombs will miss the target) = P(exactly 8 bombs will hit the target)
= P[X = 8]
= p(8)

Hence, the probability that exactly 2 bombs will miss the target = 45$$\left(\frac{2^{16}}{5^{10}}\right)$$

Question 5.
The probability that a mountain bike travelling along a certain track will have a tire burst is 0.05. Find the probability that among 17 riders:
(i) exactly one has a burst tyre
(ii) at most three have a burst tyre
(iii) two or more have burst tyres.
Solution:
Let X = number of burst tyres.
p = probability that a mountain bike travelling along a certain track will have a tyre burst.
∴ p = 0.05
∴ q = 1 – p = 1 – 0.05 = 0.95
Given: n = 17
∴ X ~ B(17, 0.05)
The p.m.f. of X is given by
P(X = x) = $${ }^{n} \mathrm{C}_{x} P^{x} q^{n-x}$$
i.e.(x) = $${ }^{17} \mathrm{C}_{x}(0.05)^{x}(0.95)^{17-x}$$, x = 0, 1, 2, ……, 17
(i) P(exactly one has a burst tyre)
P(X = 1) = p(1) = $${ }^{17} \mathrm{C}_{1}$$ (0.05)1 (0.95)17-1
= 17(0.05) (0.95)16
= 0.85(0.95)16
Hence, the probability that riders has exactly one burst tyre = (0.85)(0.95)16

(ii) P(at most three have a burst tyre) = P(X ≤ 3)
= P(X = 0) + P(X = 1) + P(X = 2) + P(X = 3)
= p(0) + p(1) + p(2) + p(3)

Hence, the probability that at most three riders have burst tyre = (2.0325)(0.95)14.

(iii) P(two or more have tyre burst) = P(X ≥ 2)
= 1 – P(X < 2)
= 1 – [P(X = 0) + P(X = 1)]
= 1 – [p(0) + p(1)]
= 1 – [$${ }^{17} \mathrm{C}_{0}$$ (0.05)0 (0.95)17 + $${ }^{17} \mathrm{C}_{1}$$ (0.05)(0.95)16]
= 1 – [1(1)(0.95)17 + 17(0.05)(0.95)16]
= 1 – (0.95)16[0.95 + 0.85]
= 1 – (1.80)(0.95)16
= 1 – (1.8)(0.95)16
Hence, the probability that two or more riders have tyre burst = 1 – (1.8)(0.95)16.

Question 6.
The probability that a lamp in a classroom will be burnt out is 0.3. Six such lamps are fitted in the classroom. If it is known that the classroom is unusable if the number of lamps burning in it is less than four, find the probability that the classroom cannot be used on a random occasion.
Solution:
Let X = number of lamps burnt out in the classroom.
p = probability of a lamp in a classroom will be burnt
∴ p = 0.3 = $$\frac{3}{10}$$
∴ q = 1 – p = 1 – $$\frac{3}{10}$$ = $$\frac{7}{10}$$
Given: n = 6
∴ X ~ B(6, $$\frac{3}{10}$$)
The p.m.f. of X is given as:
P[X = x] = $${ }^{n} \mathrm{C}_{x} p^{x} q^{n-x}$$
i.e., p(x) = $${ }^{6} \mathrm{C}_{x}\left(\frac{3}{10}\right)^{x}\left(\frac{7}{10}\right)^{6-x}$$
Since the classroom is unusable if the number of lamps burning in it is less than four, therefore
P(classroom cannot be used) = P[X < 4]
= P[X = 0] + P[X = 1] + P[X = 2] + P[X = 3]
= p(0) + p(1) + p(2) + p(3)

Hence, the probability that the classroom cannot be used on a random occasion is 0.92953.

Question 7.
A lot of 100 items contain 10 defective items. Five items are selected at random from the lot and sent to the retail store. What is the probability that the store will receive at most one defective item?
Solution:
Let X = number of defective items.
p = probability that item is defective
∴ p = $$\frac{10}{100}=\frac{1}{10}$$
∴ q = 1 – p = 1 – $$\frac{1}{10}$$ = $$\frac{9}{10}$$
Given: n = 5
∴ X ~ B(5, $$\frac{1}{10}$$)
The p.m.f. of X is given as:
P[X = x] = $${ }^{n} \mathrm{C}_{x} p^{x} q^{n-x}$$
i.e., p(x) = $${ }^{5} C_{x}\left(\frac{1}{10}\right)^{x}\left(\frac{9}{10}\right)^{5-x}$$
P (store will receive at most one defective item) = P[X ≤ 1]
=P[X = 0] + P[X = 1]
= p(0) + p(1)

Hence, the probability that the store will receive at most one defective item is (1.4)(0.9)4.

Question 8.
A large chain retailer purchases a certain kind of electronic device from a manufacturer. The manufacturer indicates that the defective rate of the device is 3%. The inspector of the retailer picks 20 items from a shipment. What is the probability that the store will receive at most one defective item?
Solution:
Let X = number of defective electronic devices.
p = probability that device is defective
∴ p = 3% = $$\frac{3}{100}$$
∴ q = 1 – p = 1 – $$\frac{3}{100}$$ = $$\frac{97}{100}$$
Given: n = 20
∴ X ~ B(20, $$\frac{3}{100}$$)
The p.m.f. of X is given as:

Hence, the probability that the store will receive at most one defective item = (1.57)(0.97)19.

Question 9.
The probability that a certain kind of component will survive a check test is 0.6. Find the probability that exactly 2 of the next 4 tested components tested survive.
Solution:
Let X = number of tested components survive.
p = probability that the component survives the check test

Hence, the probability that exactly 2 of the 4 tested components survive is 0.3456.

Question 10.
An examination consists of 10 multiple choice questions, in each of which a candidate has to deduce which one of five suggested answers is correct. A completely unprepared student guesses each answer completely randomly. What is the probability that this student gets 8 or more questions correct? Draw the appropriate moral.
Solution:
Let X = number of correct answers.
p = probability that student gets correct answer
∴ p = $$\frac{1}{5}$$
∴ q = 1 – p = 1 – $$\frac{1}{5}$$ = $$\frac{4}{5}$$
Given: n = 10 (number of total questions)
∴ X ~ B(10, $$\frac{1}{5}$$)
The p.m.f. of X is given by

Hence, the probability that student gets 8 or more questions correct = $$\frac{30.44}{5^{8}}$$

Question 11.
The probability that a machine will produce all bolts in a production run within specification is 0.998. A sample of 8 machines is taken at random. Calculate the probability that (i) all 8 machines (ii) 7 or 8 machines (iii) at most 6 machines will produce all bolts within specification.
Solution:
Let X = number of machines which produce the bolts within specification.
p = probability that a machine produce bolts within specification
p = 0.998 and q = 1 – p = 1 – 0.998 = 0.002
Given: n = 8
∴ X ~ B(8, 0.998)
The p.m.f. of X is given by
P(X = x) = $${ }^{n} \mathrm{C}_{x} p^{x} q^{n-x}$$
i.e. p(x) = $${ }^{8} \mathrm{C}_{x}(0.998)^{x}(0.002)^{8-x}$$, x = 0, 1, 2, …, 8
(i) P(all 8 machines will produce all bolts within specification) = P[X = 8]
= p(8)
= $${ }^{8} \mathrm{C}_{8}$$ (0.998)8 (0.002)8-8
= 1(0.998)8 . (1)
= (0.998)8
Hence, the probability that all 8 machines produce all bolts with specification = (0.998)8.

(ii) P(7 or 8 machines will produce all bolts within i specification) = P (X = 7) + P (X = 8)

Hence, the probability that 7 or 8 machines produce all bolts within specification = (1.014)(0.998)7.

(iii) P(at most 6 machines will produce all bolts with specification) = P[X ≤ 6]
= 1 – P[x > 6]
= 1 – [P(X = 7) + P(X = 8)]
= 1 – [P(7) + P(8)]
= 1 – (1.014)(0.998)7
Hence, the probability that at most 6 machines will produce all bolts with specification = 1 – (1.014)(0.998)7.

Question 12.
The probability that a machine develops a fault within the first 3 years of use is 0.003. If 40 machines are selected at random, calculate the probability that 38 or more will develop any faults within the first 3 years of use.
Solution:
Let X = the number of machines who develop a fault.
p = probability that a machine develops a fait within the first 3 years of use
∴ p = 0.003 and q = 1 – p = 1 – 0.003 = 0.997
Given: n = 40
∴ X ~ B(40, 0.003)
The p.m.f. of X is given by

Hence, the probability that 38 or more machines will develop the fault within 3 years of use = (775.44)(0.003)38.

Question 13.
A computer installation has 10 terminals. Independently, the probability that anyone terminal will require attention during a week is 0.1. Find the probabilities that (i) 0 (ii) 1 (iii) 2 (iv) 3 or more, terminals will require attention during the next week.
Solution:
Let X = number of terminals which required attention during a week.
p = probability that any terminal will require attention during a week
∴ p = 0.1 and q = 1 – p = 1 – 0.1 = 0.9
Given: n = 10
∴ X ~ B(10, 0.1)
The p.m.f. of X is given by
P(X = x) = $${ }^{n} \mathrm{C}_{x} p^{x} q^{n-x}$$
i.e. p(x) = $${ }^{10} C_{x}(0.1)^{x}(0.9)^{10-x}$$, x = 0, 1, 2, …, 10
(i) P(no terminal will require attention) = P(X = 0)

Hence, the probability that no terminal requires attention = (0.9)10

(ii) P(1 terminal will require attention)

Hence, the probability that 1 terminal requires attention = (0.9)9.

(iii) P(2 terminals will require attention)

Hence, the probability that 2 terminals require attention = (0.45)(0.9)8.

(iv) P(3 or more terminals will require attention)

Hence, the probability that 3 or more terminals require attention = 1 – (2.16) × (0.9)8.

Question 14.
In a large school, 80% of the pupil like Mathematics. A visitor to the school asks each of 4 pupils, chosen at random, whether they like Mathematics.
(i) Calculate the probabilities of obtaining an answer yes from 0, 1, 2, 3, 4 of the pupils.
(ii) Find the probability that the visitor obtains answer yes from at least 2 pupils:
(a) when the number of pupils questioned remains at 4.
(b) when the number of pupils questioned is increased to 8.
Solution:
Let X = number of pupils like Mathematics.
p = probability that pupils like Mathematics

(i) The probabilities of obtaining an answer yes from 0, 1, 2, 3, 4 of pupils are P(X = 0), P(X = 1), P(X = 2), P(X = 3) and P(X = 4) respectively

(ii) (a) P(visitor obtains the answer yes from at least 2 pupils when the number of pupils questioned remains at 4) = P(X ≥ 2)
= P(X = 2) + P(X = 3) + P(X = 4)

(b) P(the visitor obtains the answer yes from at least 2 pupils when number of pupils questioned is increased to 8)

Question 15.
It is observed that it rains 12 days out of 30 days. Find the probability that
(i) it rains exactly 3 days of the week.
(ii) it will rain at least 2 days of a given week.
Solution:
Let X = the number of days it rains in a week.
p = probability that it rains

(i) P(it rains exactly 3 days of week) = P(X = 3)

Hence, the probability that it rains exactly 3 days of week = 0.2903.

(ii) P(it will rain at least 2 days of the given week)

Hence, the probability that it rains at least 2 days of a given week = 0.8414

Question 16.
If the probability of success in a single trial is 0.01. How many trials are required in order to have a probability greater than 0.5 of getting at least one success?
Solution:
Let X = number of successes.
p = probability of success in a single trial
∴ p = 0.01
and q = 1 – p = 1 – 0.01 = 0.99
∴ X ~ B(n, 0.01)
The p.m.f. of X is given by
P(X = x) = $${ }^{n} \mathrm{C}_{x} p^{x} q^{n-x}$$

Hence, the number of trials required in order to have a probability greater than 0.5 of getting at least one success is $$\frac{\log 0.5}{\log 0.99}$$ or 68.

Question 17.
In binomial distribution with five Bernoulli’s trials, the probability of one and two success are 0.4096 and 0.2048 respectively. Find the probability of success.
Solution:
Given: X ~ B(n = 5, p)
The probability of X success is

Hence, the probability of success is $$\frac{1}{5}$$.

## Maharashtra Board 12th Maths Solutions Chapter 8 Binomial Distribution Ex 8.1

Balbharti 12th Maharashtra State Board Maths Solutions Book Pdf Chapter 8 Binomial Distribution Ex 8.1 Questions and Answers.

## Maharashtra State Board 12th Maths Solutions Chapter 8 Binomial Distribution Ex 8.1

Question 1.
A die is thrown 6 times. If ‘getting an odd number’ is a success, find the probability of
(i) 5 successes
(ii) at least 5 successes
(iii) at most 5 successes.
Solution:
Let X = number of successes, i.e. number of odd numbers.
p = probability of getting an odd number in a single throw of a die
∴ p = $$\frac{3}{6}=\frac{1}{2}$$ and q = 1 – p = 1 – $$\frac{1}{2}$$ = $$\frac{1}{2}$$
Given: n = 6
∴ X ~ B(6, $$\frac{1}{2}$$)
The p.m.f. of X is given by

Hence, the probability of 5 successes is $$\frac{3}{32}$$.

(ii) P(at least 5 successes) = P[X ≥ 5]
= p(5) + p(6)

Hence, the probability of at least 5 successes is $$\frac{7}{64}$$.

(iii) P(at most 5 successes) = P[X ≤ 5]
= 1 – P[X > 5]

Hence, the probability of at most 5 successes is $$\frac{63}{64}$$.

Question 2.
A pair of dice is thrown 4 times. If getting a doublet is considered a success, find the probability of two successes.
Solution:
Let X = number of doublets.
p = probability of getting a doublet when a pair of dice is thrown
∴ p = $$\frac{6}{36}=\frac{1}{6}$$ and
q = 1 – p = 1 – $$\frac{1}{6}$$ = $$\frac{5}{6}$$
Given: n = 4
∴ X ~ B(4, $$\frac{1}{6}$$)
The p.m.f. of X is given by

Hence, the probability of two successes is $$\frac{25}{216}$$.

Question 3.
There are 5% defective items in a large bulk of items. What is the probability that a sample of 10 items will include not more than one defective item?
Solution:
Let X = number of defective items.
p = probability of defective item
∴ p = 5% = $$\frac{5}{100}=\frac{1}{20}$$
and q = 1 – p = 1 – $$\frac{1}{20}$$ = $$\frac{19}{20}$$
∴ X ~ B(10, $$\frac{1}{20}$$)
The p.m.f. of X is given by

P(sample of 10 items will include not more than one defective item) = P[X ≤ 1]

Hence, the probability that a sample of 10 items will include not more than one defective item = 29$$\left(\frac{19^{9}}{20^{10}}\right)$$.

Question 4.
Five cards are drawn successively with replacement from a well-shuffled deck of 52 cards, find the probability that
(i) all the five cards are spades
(ii) only 3 cards are spades
(iii) none is a spade.
Solution:
Let X = number of spade cards.
p = probability of drawing a spade card from a pack of 52 cards.
Since there are 13 spade cards in the pack of 52 cards.
∴ p = $$\frac{13}{52}=\frac{1}{4}$$ and
q = 1 – p = 1 – $$\frac{1}{4}$$ = $$\frac{3}{4}$$
Given: n = 5
∴ X ~ B(5, $$\frac{1}{4}$$)
The p.m.f. of X is given by

(i) P(all five cards are spade)

Hence, the probability of all the five cards are spades = $$\frac{1}{1024}$$

(ii) P(only 3 cards are spade) = P[X = 3]

Hence, the probability of only 3 cards are spades = $$\frac{45}{512}$$

(iii) P(none of cards is spade) = P[X = 0]

Hence, the probability of none of the cards is a spade = $$\frac{243}{1024}$$

Question 5.
The probability of a bulb produced by a factory will fuse after 150 days of use is 0.05. Find the probability that out of 5 such bulbs
(i) none
(ii) not more than one
(iii) more than one
(iv) at least one, will fuse after 150 days of use.
Solution:
Let X = number of fuse bulbs.
p = probability of a bulb produced by a factory will fuse after 150 days of use.
∴ p = 0.05
and q = 1 – p = 1 – 0.05 = 0.95
Given: n = 5
∴ X ~ B(5, 0.05)
The p.m.f. of X is given by
P(X = x) = $${ }^{n} \mathrm{C}_{x} p^{x} q^{n-x}$$
i.e. p(x) = $${ }^{5} C_{x}(0.05)^{x}(0.95)^{5-x}$$, x = 0, 1, 2, 3, 4, 5
(i) P(none of a bulb produced by a factory will fuse after 150 days of use) = P[X = 0]
= p(0)
= $${ }^{5} \mathrm{C}_{0}(0.05)^{0}(0.95)^{5-0}$$
= 1 × 1 × (0.95)5
= (0.95)5
Hence, the probability that none of the bulbs will fuse after 150 days = (0.95)5.

(ii) P(not more than one bulb will fuse after 150 days of j use) = P[X ≤ 1]
= p(0) + p(1)
= $${ }^{5} \mathrm{C}_{0} \cdot(0.05)^{0}(0.95)^{5-0}+{ }^{5} \mathrm{C}_{1}(0.05)^{1}(0.95)^{4}$$
= 1 × 1 × (0.95)5 + 5 × (0.05) × (0.95)4
= (0.95)4 [0.95 + 5(0.05)]
= (0.95)4 (0.95 + 0.25)
= (0.95)4 (1.20)
= (1.2) (0.95)4
Hence, the probability that not more than one bulb will fuse after 150 days = (1.2)(0.95)4.

(iii) P(more than one bulb fuse after 150 days)
= P[X > 1]
= 1 – P[X ≤ 1]
= 1 – (1.2)(0.95)4
Hence, the probability that more than one bulb fuse after 150 days = 1 – (1.2)(0.95)4.

(iv) P(at least one bulb fuse after 150 days)
= P[X ≥ 1]
= 1 – P[X = 0]
= 1 – p(0)
= 1 – $${ }^{5} C_{0}(0.05)^{0}(0.95)^{5-0}$$
= 1 – 1 × 1 × (0.95)5
= 1 – (0.95)5
Hence, the probability that at least one bulb fuses after 150 days = 1 – (0.95)5.

Question 6.
A bag consists of 10 balls each marked with one of the digits 0 to 9. If four balls are drawn successively with replacement from the bag, what is the probability that none is marked with the digit 0?
Solution:
Let X = number of balls marked with digit 0.
p = probability of drawing a ball from 10 balls marked with the digit 0.
∴ p = $$\frac{1}{10}$$
and q = 1 – p = 1 – $$\frac{1}{10}$$ = $$\frac{9}{10}$$
The p.m.f. of X is given by

P(none of the ball marked with digit 0) = P(X = 0)

Hence, the probability that none of the bulb marked with digit 0 is $$\left(\frac{9}{10}\right)^{4}$$

Question 7.
On a multiple-choice examination with three possible answers for each of the five questions. What is the probability that a candidate would get four or more correct answers just by guessing?
Solution:
Let X = number of correct answers.
p = probability that a candidate gets a correct answer from three possible answers.
∴ p = $$\frac{1}{3}$$ and q = 1 – p = 1 – $$\frac{1}{3}$$ = $$\frac{2}{3}$$
Given: n = 5
∴ X ~ B(5, $$\frac{1}{3}$$)
The p.m.f. of X is given by

P(four or more correct answers) = P[X ≥ 4]
= p(4) + p(5)

Hence, the probability of getting four or more correct answers = $$\frac{11}{243}$$.

Question 8.
A person buys a lottery ticket in 50 lotteries, in each of which his chance of winning a prize is $$\frac{1}{100}$$, find the probability that he will win a prize
(i) at least once
(ii) exactly once
(iii) at least twice.
Solution:
Let X = number of winning prizes.
p = probability of winning a prize
∴ p = $$\frac{1}{100}$$
and q = 1 – p = 1 – $$\frac{1}{100}$$ = $$\frac{99}{100}$$
Given: n = 50
∴ X ~ B(50, $$\frac{1}{100}$$)
The p.m.f. of X is given by
$$P(X=x)={ }^{n} C_{x} p^{x} q^{n-x}$$
i.e., p(x) = $${ }^{50} \mathrm{C}_{x}\left(\frac{1}{100}\right)^{x}\left(\frac{99}{100}\right)^{50-x}$$, x = 0, 1, 2,… 50
(i) P(a person wins a prize at least once)

Hence, probability of winning a prize at least once = 1 – $$\left(\frac{99}{100}\right)^{50}$$

(ii) P(a person wins exactly one prize) = P[X = 1] = p(1)

Hence, probability of winning a prize exactly once = $$\frac{1}{2}\left(\frac{99}{100}\right)^{49}$$

(iii) P(a persons wins the prize at least twice) = P[X ≥ 2]
= 1 – P[X < 2]
= 1 – [p(0) + p(1)]

Hence, the probability of winning the prize at least twice = 1 – 149$$\left(\frac{99^{49}}{100^{50}}\right)$$.

Question 9.
In a box of floppy discs, it is known that 95% will work. A sample of three of the discs is selected at random. Find the probability that (i) none (ii) 1 (iii) 2 (iv) all 3 of the sample will work.
Solution:
Let X = number of working discs.
p = probability that a floppy disc works
∴ p = 95% = $$\frac{95}{100}=\frac{19}{20}$$
and q = 1 – p = 1 – $$\frac{19}{20}$$ = $$\frac{1}{20}$$
Given: n = 3
∴ X ~ B(3, $$\frac{19}{20}$$)
The p.m.f. of X is given by

(i) P(none of the floppy discs work) = P(X = 0)

Hence, the probability that none of the floppy disc will work = $$\frac{1}{20^{3}}$$.

(ii) P(exactly one floppy disc works) = P(X = 1)

Hence, the probability that exactly one floppy disc works = 3$$\left(\frac{19}{20^{3}}\right)$$

(iii) P(exactly two floppy discs work) = P(X = 2)

Hence, the probability that exactly 2 floppy discs work = 3$$\left(\frac{19^{2}}{20^{3}}\right)$$

(iv) P(all 3 floppy discs work) = P(X = 3)

Hence, the probability that all 3 floppy discs work = $$\left(\frac{19}{20}\right)^{3}$$.

Question 10.
Find the probability of throwing at most 2 sixes in 6 throws of a single die.
Solution:
Let X = number of sixes.
p = probability that a die shows six in a single throw
∴ p = $$\frac{1}{6}$$
and q = 1 – p = 1 – $$\frac{1}{6}$$ = $$\frac{5}{6}$$
Given: n = 6
∴ X ~ B(6, $$\frac{1}{6}$$)
The p.m.f. of X is given by

Hence, probability of throwing at most 2 sixes = $$\frac{7}{3}\left(\frac{5}{6}\right)^{5}$$.

Question 11.
It is known that 10% of certain articles manufactured are defective. What is the probability that in a random sample of 12 such articles, 9 are defective?
Solution:
Let X = number of defective articles.
p = probability of defective articles.
∴ p = 10% = $$\frac{10}{100}=\frac{1}{10}$$
and q = 1 – p = 1 – $$\frac{1}{10}$$ = $$\frac{9}{10}$$
Given: n = 12
∴ X ~ B(12, $$\frac{1}{10}$$)
The p.m.f. of X is given by

Hence, the probability of getting 9 defective articles = $$22\left(\frac{9^{3}}{10^{11}}\right)$$

Question 12.
Given X ~ B(n, P)
(i) If n = 10 and p = 0.4, find E(x) and Var(X).
(ii) If p = 0.6 and E(X) = 6, find n and Var(X).
(iii) If n = 25, E(X) = 10, find p and SD(X).
(iv) If n = 10, E(X) = 8, find Var(X).
Solution:
(i) Given: n = 10 and p = 0.4
∴ q = 1 – p = 1 – 0.4 = 0.6
∴ E(X) = np = 10(0.4) = 4
Var(X) = npq = 10(0.4)(0.6) = 2.4
Hence, E(X) = 4, Var(X) = 2.4.

(ii) Given: p = 0.6, E (X) = 6
E(X) = np
6 = n(0.6)
n = $$\frac{6}{0.6}$$ = 10
Now, q = 1 – p = 1 – 0.6 = 0.4
∴ Var(X) = npq = 10(0.6)(0.4) = 2.4
Hence, n = 10 and Var(X) = 2.4.

(iii) Given: n = 25, E(X) = 10
E(X) = np
10 = 25p
p = $$\frac{10}{25}=\frac{2}{5}$$
∴ q = 1 – p = 1 – $$\frac{2}{5}$$ = $$\frac{3}{5}$$
Var(X) = npq = $$25 \times \frac{2}{5} \times \frac{3}{5}$$ = 6
∴ SD(X) = √Var(X) = √6
Hence, p = $$\frac{2}{5}$$ and S.D.(X) = √6.

(iv) Given: n = 10, E(X) = 8
E(X) = np
8 = 10p
p = $$\frac{8}{10}=\frac{4}{5}$$
q = 1 – p = 1 – $$\frac{4}{5}$$ = $$\frac{1}{5}$$
Var(X) = npq = $$10\left(\frac{4}{5}\right)\left(\frac{1}{5}\right)=\frac{8}{5}$$
Hence, Var(X) = $$\frac{8}{5}$$.

## Maharashtra Board 12th Maths Solutions Chapter 7 Probability Distributions Miscellaneous Exercise 7

Balbharti 12th Maharashtra State Board Maths Solutions Book Pdf Chapter 7 Probability Distributions Miscellaneous Exercise 7 Questions and Answers.

## Maharashtra State Board 12th Maths Solutions Chapter 7 Probability Distributions Miscellaneous Exercise 7

(I) Choose the correct option from the given alternatives:

Question 1.
P.d.f. of a c.r.v. X is f(x) = 6x(1 – x), for 0 ≤ x ≤ 1 and = 0, otherwise (elsewhere) If P(X < a) = P(X > a), then a =
(a) 1
(b) $$\frac{1}{2}$$
(c) $$\frac{1}{3}$$
(d) $$\frac{1}{4}$$
(b) $$\frac{1}{2}$$

Question 2.
If the p.d.f. of a c.r.v. X is f(x) = 3(1 – 2x2), for 0 < x < 1 and = 0, otherwise (elsewhere), then the c.d.f. of X is F(x) =
(a) 2x – 3x2
(b) 3x – 4x3
(c) 3x – 2x3
(d) 2x3 – 3x
(c) 3x – 2x3

Question 3.
If the p.d.f. of a c.r.v. X is f(x) = $$\frac{x^{2}}{18}$$, for -3 < x < 3 and = 0, otherwise, then P(|X| < 1) =
(a) $$\frac{1}{27}$$
(b) $$\frac{1}{28}$$
(c) $$\frac{1}{29}$$
(d) $$\frac{1}{26}$$
(a) $$\frac{1}{27}$$

Question 4.
If p.m.f. of a d.r.v. X takes values 0, 1, 2, 3, … which probability P(X = x) = k(x +1) . 5-x, where k is a constant, then P(X = 0) =
(a) $$\frac{7}{25}$$
(b) $$\frac{16}{25}$$
(c) $$\frac{18}{25}$$
(d) $$\frac{19}{25}$$
(b) $$\frac{16}{25}$$

Question 5.
If p.m.f. of a d.r.v. X is P(X = x) = $$\frac{\left({ }^{5} \mathrm{C}_{x}\right)}{2^{5}}$$, for x = 0, 1, 2, 3, 4, 5 and = 0, otherwise. If a = P(X ≤ 2) and b = P(X ≥ 3), then
(a) a < b
(b) a > b
(c) a = b
(d) a + b
(c) a = b

Question 6.
If p.m.f. of a d.r.v. X is P(X = x) = $$\frac{x}{n(n+1)}$$, for x = 1, 2, 3, ……, n and = 0, otherwise, then E(X) =
(a) $$\frac{n}{1}+\frac{1}{2}$$
(b) $$\frac{n}{3}+\frac{1}{6}$$
(c) $$\frac{n}{2}+\frac{1}{5}$$
(d) $$\frac{n}{1}+\frac{1}{3}$$
(b) $$\frac{n}{3}+\frac{1}{6}$$

Question 7.
If p.m.f. of a d.r.v. X is P(x) = $$\frac{c}{x^{3}}$$, for x = 1, 2, 3 and = 0, otherwise (elsewhere), then E(X) =
(a) $$\frac{343}{297}$$
(b) $$\frac{294}{251}$$
(c) $$\frac{297}{294}$$
(d) $$\frac{294}{297}$$
(b) $$\frac{294}{251}$$

Question 8.
If the d.r.v. X has the following probability distribution:

then P(X = -1) =
(a) $$\frac{1}{10}$$
(b) $$\frac{2}{10}$$
(c) $$\frac{3}{10}$$
(d) $$\frac{4}{10}$$
(a) $$\frac{1}{10}$$

Question 9.
If the d.r.v. X has the following probability distribution:

then k =
(a) $$\frac{1}{7}$$
(b) $$\frac{1}{8}$$
(c) $$\frac{1}{9}$$
(d) $$\frac{1}{10}$$
(d) $$\frac{1}{10}$$

Question 10.
Find the expected value of X for the following p.m.f.

(a) 0.85
(b) -0.35
(c) 0.15
(d) -0.15
(b) -0.35

(II) Solve the following:

Question 1.
Identify the random variable as either discrete or continuous in each of the following. If the random variable is discrete, list its possible values:
(i) An economist is interested in the number of unemployed graduates in the town of population 1 lakh.
(ii) Amount of syrup prescribed by a physician.
(iii) The person on a high protein diet is interesting to gain weight in a week.
(iv) 20 white rats are available for an experiment. Twelve rats are males. A scientist randomly selects 5 rats, the number of female rats selected on a specific day.
(v) A highway-safety group is interested in studying the speed (in km/hr) of a car at a checkpoint.
Solution:
(i) Let X = number of unemployed graduates in a town.
Since the population of the town is 1 lakh, X takes the finite values.
∴ random variable X is discrete.
Range = {0, 1, 2, …, 99999, 100000}.

(ii) Let X = amount of syrup prescribed by a physician.
Then X takes uncountable infinite values.
∴ random variable X is continuous.

(iii) Let X = gain of weight in a week
Then X takes uncountable infinite values
∴ random variable X is continuous.

(iv) Let X = number of female rats selected on a specific day.
Since the total number of rats is 20 which includes 12 males and 8 females, X takes the finite values.
∴ random variable X is discrete.
Range = {0, 1, 2, 3, 4, 5}

(v) Let X = speed of .the car in km/hr.
Then X takes uncountable infinite values
∴ random variable X is continuous.

Question 2.
The probability distribution of discrete r.v. X is as follows:

(i) Determine the value of k.
(ii) Find P(X ≤ 4), P(2 < X < 4), P(X ≥ 3).
Solution:

Question 3.
The following is the probability distribution of X:

Find the probability that
(i) X is positive
(ii) X is non-negative
(iii) X is odd
(iv) X is even.
Solution:
(i) P(X is positive) = P(X = 1) + P(X = 2) + P(X = 3)
= 0.25 + 0.15 + 0.1
= 0.50

(ii) P(X is non-negative)
= P(X = 0) + P(X = 1) + P(X = 2) + P(X = 3)
= 0.20 + 0.25 + 0.15 + 0.1
= 0.70

(iii) P(X is odd)
= P(X = -3) + P(X = -1) + P(X = 1) + P(X = 3)
= 0.05 + 0.15 + 0.25 + 0.1
= 0.55

(iv) P(X is even)
= P(X = -2) + P(X = 0) + P(X = 2)
= 0.10 + 0.20 + 0.15
= 0.45.

Question 4.
The p.m.f. of a r.v. X is given by P(X = x) = x = $$\frac{{ }^{5} \mathrm{C}_{\mathrm{x}}}{2^{5}}$$, for x = 0, 1, 2, 3, 4, 5 and = 0, otherwise. Then show that P(X ≤ 2) = P(X ≥ 3).
Solution:
P(X ≤ 2) = P(X = 0) + P(X = 1) + P(X = 2)
= $$\frac{{ }^{5} \mathrm{C}_{0}}{2^{5}}+\frac{{ }^{5} \mathrm{C}_{1}}{2^{5}}+\frac{{ }^{5} \mathrm{C}_{2}}{2^{5}}$$
= $$\frac{{ }^{5} \mathrm{C}_{5}}{2^{5}}+\frac{{ }^{5} \mathrm{C}_{4}}{2^{5}}+\frac{{ }^{5} \mathrm{C}_{3}}{2^{5}}$$ ………${ }^{n} \mathrm{C}_{r}={ }^{n} \mathrm{C}_{n-r}$
= P(X = 5) + P(X = 4) + P(X = 3)
= P(X ≥ 3)
∴ P(X ≤ 2) = P(X ≥ 3).

Question 5.
In the p.m.f. of r.v. X

Find a and obtain c.d.f. of X.
Solution:
For p.m.f. of a r.v. X
$$\sum_{i=1}^{5} P(X=x)=1$$
∴ P(X = 1) + P(X = 2) + P(X = 3) + P(X = 4) + P(X = 5) = 1

Let F(x) be the c.d.f. of X.
Then F(x) = P(X ≤ x)
∴ F(1) = P(X ≤ 1) = P(X = 1) = $$\frac{1}{20}$$
F(2) = P(X ≤ 2) = P(X = 1) + P (X = 2)
$$=\frac{1}{20}+\frac{3}{20}=\frac{4}{20}=\frac{1}{5}$$
P(3) = P(X ≤ 3) = P(X = 1) + P(X = 2) + P(X = 3)
$$=\frac{1}{20}+\frac{3}{20}+\frac{5}{20}=\frac{9}{20}$$
F(4) = P(X ≤ 4) = P(X = 1) + P(X = 2) + P(X = 3) + P(X = 4)
$$=\frac{1}{20}+\frac{3}{20}+\frac{5}{20}+\frac{10}{20}=\frac{19}{20}$$
F(5) = P(X ≤ 5) = P(X = 1) + P(X = 2) + P(X = 3) + P(X = 4) + P(X = 5)
$$=\frac{1}{20}+\frac{3}{20}+\frac{5}{20}+\frac{10}{20}+\frac{1}{20}=\frac{20}{20}=1$$
Hence, the c.d.f. of the random variable X is as follows:

Question 6.
A fair coin is tossed 4 times. Let X denote the number of heads obtained. Write down the probability distribution of X. Also, find the formula for p.m.f. of X.
Solution:
When a fair coin is tossed 4 times then the sample space is
S = {HHHH, HHHT, HHTH, HTHH, THHH, HHTT, HTHT, HTTH, THHT, THTH, TTHH, HTTT, THTT, TTHT, TTTH, TTTT}
∴ n(S) = 16
X denotes the number of heads.
∴ X can take the value 0, 1, 2, 3, 4
When X = 0, then X = {TTTT}
∴ n (X) = 1
∴ P(X = 0) = $$\frac{n(X)}{n(S)}=\frac{1}{16}=\frac{{ }^{4} \mathrm{C}_{0}}{16}$$
When X = 1, then
X = {HTTT, THTT, TTHT, TTTH}
∴ n(X) = 4
∴ P(X = 1) = $$\frac{n(X)}{n(S)}=\frac{4}{16}=\frac{{ }^{4} C_{1}}{16}$$
When X = 2, then
X = {HHTT, HTHT, HTTH, THHT, THTH, TTHH}
∴ n(X) = 6
∴ P(X = 2) = $$\frac{n(X)}{n(S)}=\frac{6}{16}=\frac{{ }^{4} \mathrm{C}_{2}}{16}$$
When X = 3, then
X = {HHHT, HHTH, HTHH, THHH}
∴ n(X) = 4
∴ P(X = 3) = $$\frac{n(X)}{n(S)}=\frac{4}{16}=\frac{{ }^{4} C_{3}}{16}$$
When X = 4, then X = {HHHH}
∴ n(X) = 1
∴ P(X = 4) = $$\frac{n(X)}{n(S)}=\frac{1}{16}=\frac{{ }^{4} \mathrm{C}_{4}}{16}$$
∴ the probability distribution of X is as follows:

Also, the formula for p.m.f. of X is
P(x) = $$\frac{{ }^{4} \mathrm{C}_{x}}{16}$$, x = 0, 1, 2, 3, 4 and = 0, otherwise.

Question 7.
Find the probability distribution of the number of successes in two tosses of a die, where success is defined as
(i) number greater than 4
(ii) six appear on at least one die.
Solution:
When a die is tossed two times, we obtain (6 × 6) = 36 number of observations.
Let X be the random variable, which represents the number of successes.
Here, success refers to the number greater than 4.
P(X = 0) = P(number less than or equal to 4 on both the tosses)
= $$\frac{4}{6} \times \frac{4}{6}=\frac{16}{36}=\frac{4}{9}$$
P(X = 1) = P(number less than or equal to 4 on first toss and greater than 4 on second toss) + P(number greater than 4 on first toss and less than or equal to 4 on second toss)
= $$\frac{4}{6} \times \frac{2}{6}+\frac{4}{6} \times \frac{2}{6}$$
= $$\frac{8}{36}+\frac{8}{36}$$
= $$\frac{16}{36}$$
= $$\frac{4}{9}$$
P(X = 2) = P(number greater than 4 on both the tosses)
= $$\frac{2}{6} \times \frac{2}{6}=\frac{4}{36}=\frac{1}{9}$$
Thus, the probability distribution is as follows:

(ii) Here, success means six appears on at least one die.
P(Y = 0) = P(six appears on none of the dice) = $$\frac{5}{6} \times \frac{5}{6}=\frac{25}{36}$$
P(Y = 1) = P(six appears on none of the dice x six appears on at least one of the dice ) + P(six appears on none of the dice x six appears on at least one of the dice)
= $$\frac{1}{6} \times \frac{5}{6}+\frac{1}{6} \times \frac{5}{6}=\frac{5}{36}+\frac{5}{36}=\frac{10}{36}$$
P(Y = 2) = P(six appears on at least one of the dice) = $$\frac{1}{6} \times \frac{1}{6}=\frac{1}{36}$$
Thus, the required probability distribution is as follows:

Question 8.
A random variable X has the following probability distribution:

Determine:
(i) k
(ii) P(X > 6)
(iii) P(0 < X < 3).

Question 9.
The following is the c.d.f. of a r.v. X:

Find
(i) p.m.f. of X
(ii) P( -1 ≤ X ≤ 2)
(iii) P(X ≤ X > 0).
Solution:
(i) From the given table
F(-3) = 0.1, F(-2) = 0.3, F(-1) = 0.5
F(0) = 0.65, f(1) = 0.75, F(2) = 0.85
F(3) = 0.9, F(4) = 1
P(X = -3) = F(-3) = 0.1
P(X = -2) = F(-2) – F(-3) = 0.3 – 0.1 = 0.2
P(X = -1) = F(-1) – F(-2) = 0.5 – 0.3 = 0.2
P(X = 0) = F(0) – F(-1) = 0.65 – 0.5 = 0.15
P(X = 1) = F(1) – F(0) = 0.75 – 0.65 = 0.1
P(X = 2) = F(2) – F(1) = 0.85 – 0.75 = 0.1
P(X = 3) = F(3) – F(2) = 0.9 – 0.85 = 0.1
P(X = 4) = F(4) – F(3) = 1 – 0.9 = 0.1
∴ the p.m.f of X is as follows:

(ii) P(-1 ≤ X ≤ 2) = P(X = -1) + P(X = 0) + P(X = 1) + P(X = 2)
= 0.2 + 0.15 + 0.1 + 0.1
= 0.55

(iii) (X ≤ 3) ∩ (X > 0)
= { -3, -2, -1, 0, 1, 2, 3} n {1, 2, 3, 4}
= {1, 2, 3}

Question 10.
Find the expected value, variance, and standard deviation of the random variable whose p.m.f’s are given below:

Solution:
(i) We construct the following table to find the expected value, variance, and standard deviation:

(ii) We construct the following table to find the expected value, variance, and standard deviation:

(iii) We construct the following table to find the expected value, variance, and standard deviation:

(iv) We construct the following table to find the expected value, variance, and standard deviation:

Question 11.
A player tosses two wins. He wins ₹ 10 if 2 heads appear, ₹ 5 if 1 head appears and ₹ 2 if no head appears. Find the expected winning amount and variance of the winning amount.
Solution:
When a coin is tossed twice, the sample space is
S = {HH, HT, TH, HH}
Let X denote the amount he wins.
Then X takes values 10, 5, 2.
P(X = 10) = P(2 heads appear) = $$\frac{1}{4}$$
P(X = 5) = P(1 head appears) = $$\frac{2}{4}$$ = $$\frac{1}{2}$$
P(X = 2) = P(no head appears) = $$\frac{1}{4}$$
We construct the following table to calculate the mean and the variance of X:

From the table Σxi . P(xi) = 5.5, $$\Sigma x_{i}^{2} \cdot P\left(x_{i}\right)$$ = 38.5
E(X) = Σxi . P(xi) = 5.5
Var(X) = $$\Sigma x_{i}^{2} \cdot P\left(x_{i}\right)$$ – [E(X)]2
= 38.5 – (5.5)2
= 38.5 – 30.25
= 8.25
∴ Hence, expected winning amount = ₹ 5.5 and variance of winning amount = ₹ 8.25.

Question 12.
Let the p.m.f. of r.v. X be P(x) = $$\frac{3-x}{10}$$, for x = -1, 0, 1, 2 and = 0, otherwise.
Calculate E(X) and Var(X).
Solution:
P(X) = $$\frac{3-x}{10}$$
X takes values -1, 0, 1, 2
P(X = -1) = P(-1) = $$\frac{3+1}{10}=\frac{4}{10}$$
P(X = 0) = P(0) = $$\frac{3-0}{10}=\frac{3}{10}$$
P(X = 1) = P(1) = $$\frac{3-1}{10}=\frac{2}{10}$$
P(X = 2) = P(2) = $$\frac{3-2}{10}=\frac{1}{10}$$
We construct the following table to calculate the mean and variance of X:

From the table
ΣxiP(xi) = 0 and $$\Sigma x_{i}{ }^{2} \cdot P\left(x_{i}\right)$$ = 1
E(X) = ΣxiP(xi) = 0
Var(X) = $$\Sigma x_{i}{ }^{2} \cdot P\left(x_{i}\right)$$ – [E(X)]2
= 1 – 0
= 1
Hence, E(X) = 0, Var (X) = 1.

Question 13.
Suppose the error involved in making a certain measurement is a continuous r.v. X with p.d.f.
f(x) = k(4 – x2), -2 ≤ x ≤ 2 and = 0 otherwise.
Compute
(i) P(X > 0)
(ii) P(-1 < X < 1)
(iii) P(X < -0.5 or X > 0.5).
Solution:
(i) P(X > 0)

(ii) P(-1 < X < 1)

(iii) P(X < -0.5 or X > 0.5)

Question 14.
The p.d.f. of a continuous r.v. X is given by f(x) = $$\frac{1}{2 a}$$, for 0 < x < 2a and = 0, otherwise. Show that P( X < $$\frac{a}{2}$$) = P(X > $$\frac{3a}{2}$$)
Solution:

Question 15.
The p.d.f. of r.v. X is given by f(x) = $$\frac{k}{\sqrt{x}}$$, for 0 < x < 4 and = 0, otherwise. Determine k. Determine c.d.f. of X and hence find P(X ≤ 2) and P(X ≤ 1).
Solution:
Since f is p.d.f. of the r.v. X,

## Maharashtra Board 12th Maths Solutions Chapter 7 Probability Distributions Ex 7.2

Balbharti 12th Maharashtra State Board Maths Solutions Book Pdf Chapter 7 Probability Distributions Ex 7.2 Questions and Answers.

## Maharashtra State Board 12th Maths Solutions Chapter 7 Probability Distributions Ex 7.2

Question 1.
Verify which of the following is p.d.f. of r.v. X:
(i) f(x) = sin x, for 0 ≤ x ≤ $$\frac{\pi}{2}$$
(ii) f(x) = x, for 0 ≤ x ≤ 1 and -2 – x for 1 < x < 2
(iii) fix) = 2, for 0 ≤ x ≤ 1.
Solution:
f(x) is the p.d.f. of r.v. X if
(a) f(x) ≥ 0 for all x ∈ R and

Hence, f(x) is the p.d.f. of X.

(ii) f(x) = x ≥ 0 if 0 ≤ x ≤ 1
For 1 < x < 2, -2 < -x < -1
-2 – 2 < -2 – x < -2 – 1
i.e. -4 < f(x) < -3 if 1 < x < 2
Hence, f(x) is not p.d.f. of X.

(iii) (a) f(x) = 2 ≥ 0 for 0 ≤ x ≤ 1

Hence, f(x) is not p.d.f. of X.

Question 2.
The following is the p.d.f. of r.v. X:
f(x) = $$\frac{x}{8}$$, for 0 < x < 4 and = 0 otherwise.
Find
(a) P(x < 1.5)
(b) P(1 < x < 2) (c) P(x > 2).
Solution:

Question 3.
It is known that error in measurement of reaction temperature (in 0°C) in a certain experiment is continuous r.v. given by
f(x) = $$\frac{x^{2}}{3}$$ for -1 < x < 2
= 0. otherwise.
(i) Verify whether f(x) is p.d.f. of r.v. X
(ii) Find P(0 < x ≤ 1)
(iii) Find the probability that X is negative.
Solution:

Question 4.
Find k if the following function represents p.d.f. of r.v. X
(i) f(x) = kx. for 0 < x < 2 and = 0 otherwise.
Also find P($$\frac{1}{4}$$ < x < $$\frac{3}{2}$$).
(ii) f(x) = kx(1 – x), for 0 < x < 1 and = 0 otherwise.
Also find P($$\frac{1}{4}$$ < x < $$\frac{1}{2}$$), P(x < $$\frac{1}{2}$$).
Solution:
(i) Since, the function f is p.d.f. of X

(ii) Since, the function f is the p.d.f. of X,

Question 5.
Let X be the amount of time for which a book is taken out of the library by a randomly selected students and suppose X has p.d.f.
f(x) = 0.5x, for 0 ≤ x ≤ 2 and = 0 otherwise.
Calculate:
(i) P(x ≤ 1)
(ii) P(0.5 ≤ x ≤ 1.5)
(iii) P(x ≥ 1.5).
Solution:
(i) P(x ≤ 1)

(ii) P(0.5 ≤ x ≤ 1.5)

(iii) P(x ≥ 1.5)

Question 6.
Suppose that X is waiting time in minutes for a bus and its p.d.f. is given by f(x) = $$\frac{1}{5}$$, for 0 ≤ x ≤ 5 and = 0 otherwise. Find the probability that
(i) waiting time is between 1 and 3
(ii) waiting time is more than 4 minutes.
Solution:
(i) Required probability = P(1 < X < 3)

(ii) Required probability = P(X > 4)

Question 7.
Suppose the error involved in making a certain measurement is a continuous r.v. X with p.d.f.
f(x) = k(4 – x2), -2 ≤ x ≤ 2 and 0 otherwise.
Compute:
(i) P(X > 0)
(ii) P(-1 < X < 1)
(iii) P(-0.5 < X or X > 0.5).
Solution:
Since, f is the p.d.f. of X,

Question 8.
The following is the p.d.f. of continuous r.v. X
f(x) = $$\frac{x}{8}$$, for 0 < x < 4 and = 0 otherwise.
(i) Find expression for c.d.f. of X.
(ii) Find F(x) at x = 0.5, 1.7 and 5.
Solution:
(i) Let F(x) be the c.d.f. of X

Question 9.
Given the p.d.f. of a continuous random r.v. X, f(x) = $$\frac{x^{2}}{3}$$, for -1 < x < 2 and = 0 otherwise. Determine c.d.f. of X and hence find P(X < 1); P(X < -2), P(X > 0), P(1 < X < 2).
Solution:

Question 10.
If a r.v. X has p.d.f.
f(x) = $$\frac{c}{x}$$ for 1 < x < 3, c > 0. Find c, E(X), Var (X).
Solution:
Since f(x) is p.d.f of r.v. X

## Maharashtra Board 12th Maths Solutions Chapter 7 Probability Distributions Ex 7.1

Balbharti 12th Maharashtra State Board Maths Solutions Book Pdf Chapter 7 Probability Distributions Ex 7.1 Questions and Answers.

## Maharashtra State Board 12th Maths Solutions Chapter 7 Probability Distributions Ex 7.1

Question 1.
Let X represent the difference between a number of heads and the number of tails when a coin is tossed 6 times. What are the possible values of X?
Solution:
When a coin is tossed 6 times, the number of heads can be 0, 1, 2, 3, 4, 5, 6.
The corresponding number of tails will be 6, 5, 4, 3, 2, 1, 0.
∴ X can take values 0 – 6, 1 – 5, 2 – 4, 3 – 3, 4 – 2, 5 – 1, 6 – 0
i.e. -6, -4, -2, 0, 2, 4, 6.
∴ X = {-6, -4, -2, 0, 2, 4, 6}.

Question 2.
An urn contains 5 red and 2 black balls. Two balls are drawn at random. X denotes the number of black balls drawn. What are the possible values of X?
Solution:
The urn contains 5 red and 2 black balls.
If two balls are drawn from the urn, it contains either 0 or 1 or 2 black balls.
X can take values 0, 1, 2.
∴ X = {0, 1, 2}.

Question 3.
State which of the following are not the probability mass function of a random variable. Give reasons for your answer.

Solution:
P.m.f. of random variable should satisfy the following conditions:
(a) 0 ≤ pi ≤ 1
(b) Σpi = 1.

(i)

(a) Here 0 ≤ pi ≤ 1
(b) Σpi = 0.4 + 0.4 + 0.2 = 1
Hence, P(X) can be regarded as p.m.f. of the random variable X.

(ii)

P(X = 3) = -0.1, i.e. Pi < 0 which does not satisfy 0 ≤ Pi ≤ 1
Hence, P(X) cannot be regarded as p.m.f. of the random variable X.

(iii)

(a) Here 0 ≤ pi ≤ 1
(b) ∑pi = 0.1 + 0.6 + 0.3 = 1
Hence, P(X) can be regarded as p.m.f. of the random variable X.

(iv)

Here ∑pi = 0.3 + 0.2 + 0.4 + 0 + 0.05 = 0.95 ≠ 1
Hence, P(Z) cannot be regarded as p.m.f. of the random variable Z.

(v)

Here ∑pi = 0.6 + 0.1 + 0.2 = 0.9 ≠ 1
Hence, P(Y) cannot be regarded as p.m.f. of the random variable Y.

(vi)

(a) Here 0 ≤ pi ≤ 1
(b) ∑pi = 0.3 + 0.4 + 0.3 = 1
Hence, P(X) can be regarded as p.m.f. of the random variable X.

Question 4.
Find the probability distribution of
(i) number of heads in two tosses of a coin.
(ii) number of tails in the simultaneous tosses of three coins.
(iii) number of heads in four tosses of a coin.
Solution:
(i) For two tosses of a coin the sample space is {HH, HT, TH, TT}
Let X denote the number of heads in two tosses of a coin.
Then X can take values 0, 1, 2.
∴ P[X = 0] = P(0) = $$\frac{1}{4}$$
P[X = 1] = P(1) = $$\frac{2}{4}$$ = $$\frac{1}{2}$$
P[X = 2] = P(2) = $$\frac{1}{4}$$
∴ the required probability distribution is

(ii) When three coins are tossed simultaneously, then the sample space is
{HHH, HHT, HTH, THH, HTT, THT, TTH, TTT}
Let X denotes the number of tails.
Then X can take the value 0, 1, 2, 3.
∴ P[X = 0] = P(0) = $$\frac{1}{8}$$
P[X = 1] = P(1) = $$\frac{3}{8}$$
P[X = 2] = P(2) = $$\frac{3}{8}$$
P[X = 3] = P(3) = $$\frac{1}{8}$$
∴ the required probability distribution is

(iii) When a fair coin is tossed 4 times, then the sample space is
S = {HHHH, HHHT, HHTH, HTHH, THHH, HHTT, HTHT, HTTH, THHT, THTH, TTHH, HTTT, THTT, TTHT, TTTH, TTTT}
∴ n(S) = 16
Let X denotes the number of heads.
Then X can take the value 0, 1, 2, 3, 4
When X = 0, then X = {TTTT}
∴ n(X) = 1
∴ P(X = 0) = $$\frac{n(X)}{n(S)}=\frac{1}{16}$$
When X = 1, then
X = {HTTT, THTT, TTHT, TTTH}
∴ n(X) = 4
∴ P(X = 1) = $$\frac{n(X)}{n(S)}=\frac{4}{16}=\frac{1}{4}$$
When X = 2, then
X = {HHTT, HTHT, HTTH, THHT, THTH, TTHH}
∴ n(X) = 6
∴ P(X = 2) = $$\frac{n(X)}{n(S)}=\frac{6}{16}=\frac{3}{8}$$
When X = 3, then
X = {HHHT, HHTH, HTHH, THHH}
∴ n(X) = 4
∴ P(X = 3) = $$\frac{n(X)}{n(S)}=\frac{4}{16}=\frac{1}{4}$$
When X = 4, then X = {HHHH}
∴ n(X) = 1
∴ P(X = 4) = $$\frac{n(X)}{n(S)}=\frac{1}{16}$$
∴ the probability distribution of X is as follows:

Question 5.
Find the probability distribution of a number of successes in two tosses of a die, where success is defined as a number greater than 4 appearing on at least one die.
Solution:
When a die is tossed twice, the sample space s has 6 × 6 = 36 sample points.
∴ n(S) = 36
The trial will be a success if the number on at least one die is 5 or 6.
Let X denote the number of dice on which 5 or 6 appears.
Then X can take values 0, 1, 2.
When X = 0 i.e., 5 or 6 do not appear on any of the dice, then
X = {(1, 1), (1, 2), (1, 3), (1, 4), (2, 1), (2, 2), (2, 3), (2, 4), (3, 1), (3, 2), (3, 3), (3, 4), (4, 1), (4, 2), (4, 3), (4, 4)}
∴ n(X) = 16.
∴ P(X = 0) = $$\frac{n(X)}{n(S)}=\frac{16}{36}=\frac{4}{9}$$
When X = 1, i.e. 5 or 6 appear on exactly one of the dice, then
X = {(1, 5), (1, 6), (2, 5), (2, 6), (3, 5), (3, 6), (4, 5), (4, 6), (5, 1), (5, 2), (5, 3), (5, 4), (6, 1), (6, 2), (6, 3), (6, 4)}
∴ n(X) = 16
∴ P(X = 1) = $$\frac{n(X)}{n(S)}=\frac{16}{36}=\frac{4}{9}$$
When X = 2, i.e. 5 or 6 appear on both of the dice, then
X = {(5, 5), (5, 6), (6, 5), (6, 6)}
∴ n(X) = 4
∴ P(X = 2) = $$\frac{n(X)}{n(S)}=\frac{4}{36}=\frac{1}{9}$$
∴ the required probability distribution is

Question 6.
From a lot of 30 bulbs which include 6 defectives, a sample of 4 bulbs is drawn at random with replacement. Find the probability distribution of the number of defective bulbs.
Solution:
Here, the number of defective bulbs is the random variable.
Let the number of defective bulbs be denoted by X.
∴ X can take the value 0, 1, 2, 3, 4.
Since the draws are done with replacement, therefore the four draws are independent experiments.
Total number of bulbs is 30 which include 6 defectives.
∴ P(X = 0) = P(0) = P(all 4 non-defective bulbs)
= $$\frac{24}{30} \times \frac{24}{30} \times \frac{24}{30} \times \frac{24}{30}$$
= $$\frac{256}{625}$$
P(X = 1) = P (1) = P (1 defective and 3 non-defective bulbs)

P(X = 2) = P(2) = P(2 defective and 2 non-defective)

P(X = 3) = P(3) = P(3 defectives and 1 non-defective)

P(X = 4) = P(4) = P(all 4 defectives)
= $$\frac{6}{30} \times \frac{6}{30} \times \frac{6}{30} \times \frac{6}{30}$$
= $$\frac{1}{625}$$
∴ the required probability distribution is

Question 7.
A coin is biased so that the head is 3 times as likely to occur as the tail. If the coin is tossed twice. Find the probability distribution of a number of tails.
Solution:
Given a biased coin such that heads is 3 times as likely as tails.
∴ P(H) = $$\frac{3}{4}$$ and P(T) = $$\frac{1}{4}$$
The coin is tossed twice.
Let X can be the random variable for the number of tails.
Then X can take the value 0, 1, 2.
∴ P(X = 0) = P(HH) = $$\frac{3}{4} \times \frac{3}{4}=\frac{9}{16}$$
P(X = 1) = P(HT, TH) = $$\frac{3}{4} \times \frac{1}{4}+\frac{1}{4} \times \frac{3}{4}=\frac{6}{16}=\frac{3}{8}$$
P(X = 2) = P(TT) = $$\frac{1}{4} \times \frac{1}{4}=\frac{1}{16}$$
∴ the required probability distribution is

Question 8.
A random variable X has the following probability distribution:

Determine:
(i) k
(ii) P(X < 3) (iii) P(X > 4)
Solution:
(i) Since P (x) is a probability distribution of x,
$$\sum_{x=0}^{7} P(x)=1$$
⇒ P(0) + P(1) + P(2) + P(3) + P(4) + P(5) + P(6) + P(7) = 1
⇒ 0 + k + 2k + 2k + 3k + k2 + 2k2 + 7k2 + k = 1
⇒ 10k2 + 9k – 1 = 0
⇒ 10k2 + 10k – k – 1 = 0
⇒ 10k(k + 1) – 1(k + 1) = 0
⇒ (k + 1)(10k – 1) = 0
⇒ 10k – 1 = 0 ……..[∵ k ≠ -1]
⇒ k = $$\frac{1}{10}$$

(ii) P(X< 3) = P(0) + P(1) + P(2)
= 0 + k + 2k
= 3k
= 3($$\frac{1}{10}$$)
= $$\frac{3}{10}$$

(iii) P(0 < X < 3) = P (1) + P (2)
= k + 2k
= 3k
= 3($$\frac{1}{10}$$)
= $$\frac{3}{10}$$

Question 9.
Find expected value and variance of X for the following p.m.f.:

Solution:
We construct the following table to calculate E(X) and V(X):

From the table,
Σxipi = -0.05 and $$\Sigma x_{i}^{2} \cdot p_{i}$$ = 2.25
∴ E(X) = Σxipi = -0.05
and V(X) = $$\Sigma x_{i}^{2}+p_{i}-\left(\sum x_{i}+p_{i}\right)^{2}$$
= 2.25 – (-0.05)2
= 2.25 – 0.0025
= 2.2475
Hence, E(X) = -0.05 and V(X) = 2.2475.

Question 10.
Find expected value and variance of X, where X is the number obtained on the uppermost face when a fair die is thrown.
Solution:
If a die is tossed, then the sample space for the random variable X is
S = {1, 2, 3, 4, 5, 6}
∴ P(X) = $$\frac{1}{6}$$; X = 1, 2, 3, 4, 5, 6.

Hence, E(X) = 3.5 and V(X) = 2.9167.

Question 11.
Find the mean number of heads in three tosses of a fair coin.
Solution:
When three coins are tossed the sample space is {HHH, HHT, THH, HTH, HTT, THT, TTH, TTT}
∴ n(S) = 8
Let X denote the number of heads when three coins are tossed.
Then X can take values 0, 1, 2, 3
P(X = 0) = P(0) = $$\frac{1}{8}$$
P(X = 1) = P(1) = $$\frac{3}{8}$$
P(X = 2) = P(2) = $$\frac{3}{8}$$
P(X = 3) = P(3) = $$\frac{1}{8}$$
∴ mean = E(X) = ΣxiP(xi)
= $$0 \times \frac{1}{8}+1 \times \frac{3}{8}+2 \times \frac{3}{8}+3 \times \frac{1}{8}$$
= $$0+\frac{3}{8}+\frac{6}{8}+\frac{3}{8}$$
= $$\frac{12}{8}$$
= 1.5

Question 12.
Two dice are thrown simultaneously. If X denotes the number of sixes, find the expectation of X.
Solution:
When two dice are thrown, the sample space S has 6 × 6 = 36 sample points.
∴ n(S) = 36
Let X denote the number of sixes when two dice are thrown.
Then X can take values 0, 1, 2
When X = 0, then
X = {(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (3, 1), (3, 2), (3, 3), (3, 4), (3, 5), (4, 1), (4, 2), (4, 3), (4, 4), (4, 5), (5, 1), (5, 2), (5, 3), (5, 4), (5, 5)}
∴ n(X) = 25
∴ P(X = 0) = $$\frac{n(X)}{n(S)}=\frac{25}{36}$$
When X = 1, then
X = {(1, 6), (2, 6), (3, 6), (4, 6), (5, 6), (6, 1), (6, 2), (6, 3), (6, 4), (6, 5)}
∴ n(X) = 10
∴ P(X = 1) = $$\frac{n(X)}{n(S)}=\frac{10}{36}$$
When X = 2, then X = {(6, 6)}
∴ n(X) = 1
∴ P(X = 2) = $$\frac{n(X)}{n(S)}=\frac{1}{36}$$
∴ E(X) = ΣxiP(xi)
= $$0 \times \frac{25}{36}+1 \times \frac{10}{36}+2 \times \frac{1}{36}$$
= $$0+\frac{10}{36}+\frac{2}{36}$$
= $$\frac{1}{3}$$

Question 13.
Two numbers are selected at random (without replacement) from the first six positive integers. Let X denote the larger of the two numbers. Find E(X).
Solution:
Two numbers are chosen from the first 6 positive integers.
∴ n(S) = $${ }^{6} C_{2}=\frac{6 \times 5}{1 \times 2}$$ = 15
Let X denote the larger of the two numbers.
Then X can take values 2, 3, 4, 5, 6.
When X = 2, the other positive number which is less than 2 is 1.
∴ n(X) = 1
∴ P(X = 2) = P(2) = $$\frac{n(X)}{n(S)}=\frac{1}{15}$$
When X = 3, the other positive number less than 3 can be 1 or 2 and hence can be chosen in 2 ways.
∴ n(X) = 2
P(X = 3) = P(3) = $$\frac{n(X)}{n(S)}=\frac{2}{15}$$
Similarly, P(X = 4) = P(4) = $$\frac{3}{15}$$
P(X = 5) = P(5) = $$\frac{4}{15}$$
P(X = 6) = P(6) = $$\frac{5}{15}$$
∴ E(X) = ΣxiP(xi)
= $$2 \times \frac{1}{15}+3 \times \frac{2}{15}+4 \times \frac{3}{15}+5 \times \frac{4}{15}+6 \times \frac{5}{15}$$
= $$\frac{2+6+12+20+30}{15}$$
= $$\frac{70}{15}$$
= $$\frac{14}{3}$$

Question 14.
Let X denote the sum of numbers obtained when two fair dice are rolled. Find the standard deviation of X.
Solution:
If two fair dice are rolled then the sample space S of this experiment is
S = {(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (2, 6), (3, 1), (3, 2), (3, 3), (3, 4), (3, 5), (3, 6), (4, 1), (4, 2), (4, 3), (4, 4), (4, 5), (4, 6), (5, 1), (5, 2), (5, 3), (5, 4), (5, 5), (5, 6), (6, 1), (6, 2), (6, 3), (6, 4), (6, 5), (6, 6)}
∴ n(S) = 36
Let X denote the sum of the numbers on uppermost faces.
Then X can take the values 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12

∴ the probability distribution of X is given by

Question 15.
A class has 15 students whose ages are 14, 17, 15, 14, 21, 17, 19, 20, 16, 18, 20, 17, 16, 19 and 20 years. One student is selected in such a manner that each has the same chance of being chosen and the age X of the student is recorded. What is the probability distribution of the random variable X? Find mean, variance, and standard deviation of X.
Solution:
Let X denote the age of the chosen student. Then X can take values 14, 15, 16, 17, 18, 19, 20, 21.
We make a frequency table to find the number of students with age X:

The chances of any student selected are equally likely.
If there are m students with age X, then P(X) = $$\frac{m}{15}$$
Using this, the following is the probability distribution of X:

Variance = V(X) = $$\Sigma x_{i}^{2}$$ . P(xi) – [E(X)]2
= 312.2 – (17.53)2
= 312.2 – 307.3
= 4.9
Standard deviation = √V(X) = √4.9 = 2.21
Hence, mean = 17.53, variance = 4.9 and standard deviation = 2.21.

Question 16.
In a meeting, 70% of the member’s favour and 30% oppose a certain proposal. A member is selected at random and we take X = 0 if he opposed and X = 1 if he is in favour. Find E(X) and Var(X).
Solution:
X takes values 0 and 1.
It is given that
P(X = 0) = P(0) = 30% = $$\frac{30}{100}$$ = 0.3
P(X = 1) = P(1) = 70% = $$\frac{70}{100}$$ = 0.7
∴ E(X) = Σxi . P(xi) = 0 × 0.3 + 1 × 0.7 = 0.7
Also, $$\Sigma x_{i}^{2} \cdot P\left(x_{i}\right)$$ = 0 × 0.3 + 1 × 0.7 = 0.7
∴ Variance = V(X) = $$\Sigma x_{i}^{2} \cdot P\left(x_{i}\right)-[E(X)]^{2}$$
= 0.7 – (0.7)2
= 0.7 – 0.49
= 0.21
Hence, E(X) = 0.7 and Var(X) = 0.21.

## Maharashtra Board 12th Maths Solutions Chapter 6 Differential Equations Miscellaneous Exercise 6

Balbharti 12th Maharashtra State Board Maths Solutions Book Pdf Chapter 6 Differential Equations Miscellaneous Exercise 6 Questions and Answers.

## Maharashtra State Board 12th Maths Solutions Chapter 6 Differential Equations Miscellaneous Exercise 6

(I) Choose the correct option from the given alternatives:

Question 1.
The order and degree of the differential equation $$\sqrt{1+\left(\frac{d y}{d x}\right)^{2}}=\left(\frac{d^{2} y}{d x^{2}}\right)^{\frac{3}{2}}$$ are respectively……..
(a) 2, 1
(b) 1, 2
(c) 3, 2
(d) 2, 3
(d) 2, 3

Question 2.
The differential equation of y = c2 + $$\frac{c}{x}$$ is…….
(a) $$x^{4}\left(\frac{d y}{d x}\right)^{2}-x \frac{d y}{d x}=y$$
(b) $$\frac{d y}{d x^{2}}+x \frac{d y}{d x}+y=0$$
(c) $$x^{3}\left(\frac{d y}{d x}\right)^{2}+x \frac{d y}{d x}=y$$
(d) $$\frac{d^{2} y}{d x^{2}}+\frac{d y}{d x}-y=0$$
(a) $$x^{4}\left(\frac{d y}{d x}\right)^{2}-x \frac{d y}{d x}=y$$

Question 3.
x2 + y2 = a2 is a solution of ………
(a) $$\frac{d^{2} y}{d x^{2}}+\frac{d y}{d x}-y=0$$
(b) $$y=x \sqrt{1+\left(\frac{d y}{d x}\right)^{2}}+a^{2} y$$
(c) $$y=x \frac{d y}{d x}+a \sqrt{1+\left(\frac{d y}{d x}\right)^{2}}$$
(d) $$\frac{d^{2} y}{d x^{2}}=(x+1) \frac{d y}{d x}$$
(c) $$y=x \frac{d y}{d x}+a \sqrt{1+\left(\frac{d y}{d x}\right)^{2}}$$

Question 4.
The differential equation of all circles having their centres on the line y = 5 and touching the X-axis is
(a) $$y^{2}\left(1+\frac{d y}{d x}\right)=25$$
(b) $$(y-5)^{2}\left[1+\left(\frac{d y}{d x}\right)^{2}\right]=25$$
(c) $$(y-5)^{2}+\left[1+\left(\frac{d y}{d x}\right)^{2}\right]=25$$
(d) $$(y-5)^{2}\left[1-\left(\frac{d y}{d x}\right)^{2}\right]=25$$
(b) $$(y-5)^{2}\left[1+\left(\frac{d y}{d x}\right)^{2}\right]=25$$

Question 5.
The differential equation y $$\frac{d y}{d x}$$ + x = 0 represents family of ………
(a) circles
(b) parabolas
(c) ellipses
(d) hyperbolas
(a) circles

Hint:
y $$\frac{d y}{d x}$$ + x = 0
∴ ∫y dy + ∫x dx = c
∴ $$\frac{y^{2}}{2}+\frac{x^{2}}{2}=c$$
∴ x2 + y2 = 2c which is a circle.

Question 6.
The solution of $$\frac{1}{x} \cdot \frac{d y}{d x}=\tan ^{-1} x$$ is……
(a) $$\frac{x^{2} \tan ^{-1} x}{2}+c=0$$
(b) x tan-1x + c = 0
(c) x – tan-1x = c
(d) $$y=\frac{x^{2} \tan ^{-1} x}{2}-\frac{1}{2}\left(x-\tan ^{-1} x\right)+c$$
(d) $$y=\frac{x^{2} \tan ^{-1} x}{2}-\frac{1}{2}\left(x-\tan ^{-1} x\right)+c$$

Question 7.
The solution of (x + y)2 $$\frac{d y}{d x}$$ = 1 is…….
(a) x = tan-1(x + y) + c
(b) y tan-1($$\frac{x}{y}$$) = c
(c) y = tan-1(x + y) + c
(d) y + tan-1(x + y) = c
(c) y = tan-1(x + y) + c

Question 8.
The Solution of $$\frac{d y}{d x}=\frac{y+\sqrt{x^{2}-y^{2}}}{2}$$ is……
(a) sin-1($$\frac{y}{x}$$) = 2 log |x| + c
(b) sin-1($$\frac{y}{x}$$) = log |x| + c
(c) sin($$\frac{x}{y}$$) = log |x| + c
(d) sin($$\frac{y}{x}$$) = log |y| + c
(b) sin-1($$\frac{y}{x}$$) = log |x| + c

Question 9.
The solution of $$\frac{d y}{d x}$$ + y = cos x – sin x is……
(a) y ex = cos x + c
(b) y ex + ex cos x = c
(c) y ex = ex cos x + c
(d) y2 ex = ex cos x + c
(c) y ex = ex cos x + c
Hint:
$$\frac{d y}{d x}$$ + y = cos x – sin x
I.F. = $$e^{\int 1 d x}=e^{x}$$
∴ the solution is y . ex = ∫(cos x – sin x) ex + c
∴ y . ex = ex cos x + c

Question 10.
The integrating factor of linear differential equation x $$\frac{d y}{d x}$$ + 2y = x2 log x is……..
(a) $$\frac{1}{x}$$
(b) k
(c) $$\frac{1}{n^{2}}$$
(d) x2
(d) x2
Hint:
I.F. = $$e^{\int \frac{2}{x} d x}$$
= e2 log x
= x2

Question 11.
The solution of the differential equation $$\frac{d y}{d x}$$ = sec x – y tan x is…….
(a) y sec x + tan x = c
(b) y sec x = tan x + c
(c) sec x + y tan x = c
(d) sec x = y tan x + c
(b) y sec x = tan x + c

Hint:
$$\frac{d y}{d x}$$ = sec x – y tan x
∴ $$\frac{d y}{d x}$$ + y tan x = sec x
I.F. = $$e^{\int \tan x d x}=e^{\log \sec x}$$ = sec x
∴ the solution is
y . sec x = ∫sec x . sec x dx + c
∴ y sec x = tan x + c

Question 12.
The particular solution of $$\frac{d y}{d x}=x e^{y-x}$$, when x = y = 0 is……
(a) ex-y = x + 1
(b) ex+y = x + 1
(c) ex + ey = x + 1
(d) ey-x = x – 1
(a) ex-y = x + 1

Question 13.
$$\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$$ is a solution of……..
(a) $$\frac{d^{2} y}{d x^{2}}+y x+\left(\frac{d y}{d x}\right)^{2}=0$$
(b) $$x y \frac{d^{2} y}{d x^{2}}+2\left(\frac{d y}{d x}\right)^{2}-y \frac{d y}{d x}=0$$
(c) $$y \frac{d^{2} y}{d x^{2}}+2\left(\frac{d y}{d x}\right)^{2}+y=0$$
(d) $$x y \frac{d y}{d x}+y \frac{d^{2} y}{d x^{2}}=0$$
(b) $$x y \frac{d^{2} y}{d x^{2}}+2\left(\frac{d y}{d x}\right)^{2}-y \frac{d y}{d x}=0$$

Question 14.
The decay rate of certain substances is directly proportional to the amount present at that instant. Initially, there are 27 grams of substance and 3 hours later it is found that 8 grams left. The amount left after one more hour is……
(a) 5$$\frac{2}{3}$$ grams
(b) 5$$\frac{1}{3}$$ grams
(c) 5.1 grams
(d) 5 grams
(b) 5$$\frac{1}{3}$$ grams

Question 15.
If the surrounding air is kept at 20°C and the body cools from 80°C to 70°C in 5 minutes, the temperature of the body after 15 minutes will be…..
(a) 51.7°C
(b) 54.7°C
(c) 52.7°C
(d) 50.7°C
(b) 54.7°C

(II) Solve the following:

Question 1.
Determine the order and degree of the following differential equations:
(i) $$\frac{d^{2} y}{d x^{2}}+5 \frac{d y}{d x}+y=x^{3}$$
Solution:
The given D.E. is $$\frac{d^{2} y}{d x^{2}}+5 \frac{d y}{d x}+y=x^{3}$$
This D.E. has highest order derivative $$\frac{d^{2} y}{d x^{2}}$$ with power 1.
∴ the given D.E. is of order 2 and degree 1.

(ii) $$\left(\frac{d^{3} y}{d x^{3}}\right)^{2}=\sqrt[5]{1+\frac{d y}{d x}}$$
Solution:
The given D.E. is $$\left(\frac{d^{3} y}{d x^{3}}\right)^{2}=\sqrt[5]{1+\frac{d y}{d x}}$$
$$\left(\frac{d^{3} y}{d x^{3}}\right)^{2 \times 5}=1+\frac{d y}{d x}$$
$$\left(\frac{d^{3} y}{d x^{3}}\right)^{10}=1+\frac{d y}{d x}$$
This D.E. has highest order derivative $$\frac{d^{3} y}{d x^{3}}$$ with power 10.
∴ the given D.E. is of order 3 and degree 10.

(iii) $$\sqrt[3]{1+\left(\frac{d y}{d x}\right)^{2}}=\frac{d^{2} y}{d x^{2}}$$
Solution:
The given D.E. is $$\sqrt[3]{1+\left(\frac{d y}{d x}\right)^{2}}=\frac{d^{2} y}{d x^{2}}$$
On cubing both sides, we get
$$1+\left(\frac{d y}{d x}\right)^{2}=\left(\frac{d^{2} y}{d x^{2}}\right)^{3}$$
This D.E. has highest order derivative $$\frac{d^{2} y}{d x^{2}}$$ with power 3.
∴ the given D.E. is of order 2 and degree 3.

(iv) $$\frac{d y}{d x}=3 y+\sqrt[4]{1+5\left(\frac{d y}{d x}\right)^{2}}$$
Solution:
The given D.E. is

This D.E. has the highest order derivative $$\frac{d y}{d x}$$ with power 4.
∴ the given D.E. is of order 1 and degree 4.

(v) $$\frac{d^{4} y}{d x^{4}}+\sin \left(\frac{d y}{d x}\right)=0$$
Solution:
The given D.E. is $$\frac{d^{4} y}{d x^{4}}+\sin \left(\frac{d y}{d x}\right)=0$$
This D.E. has highest order derivative $$\frac{d^{4} y}{d x^{4}}$$.
∴ order = 4
Since this D.E. cannot be expressed as a polynomial in differential coefficient, the degree is not defined.

Question 2.
In each of the following examples verify that the given function is a solution of the differential equation.
(i) $$x^{2}+y^{2}=r^{2} ; x \frac{d y}{d x}+r \sqrt{1+\left(\frac{d y}{d x}\right)^{2}}=y$$
Solution:
x2 + y2 = r2 ……. (1)
Differentiating both sides w.r.t. x, we get

Hence, x2 + y2 = r2 is a solution of the D.E.
$$x \frac{d y}{d x}+r \sqrt{1+\left(\frac{d y}{d x}\right)^{2}}=y$$

(ii) y = eax sin bx; $$\frac{d^{2} y}{d x^{2}}-2 a \frac{d y}{d x}+\left(a^{2}+b^{2}\right) y=0$$
Solution:

(iii) y = 3 cos(log x) + 4 sin(log x); $$x^{2} \frac{d^{2} y}{d x^{2}}+x \frac{d y}{d x}+y=0$$
Solution:
y = 3 cos(log x) + 4 sin (log x) …… (1)
Differentiating both sides w.r.t. x, we get

(iv) xy = aex + be-x + x2; $$x \frac{d^{2} y}{d x^{2}}+2 \frac{d y}{d x}+x^{2}=x y+2$$
Solution:

(v) x2 = 2y2 log y, x2 + y2 = xy $$\frac{d x}{d y}$$
Solution:
x2 = 2y2 log y ……(1)
Differentiating both sides w.r.t. y, we get

∴ x2 + y2 = xy $$\frac{d x}{d y}$$
Hence, x2 = 2y2 log y is a solution of the D.E.
x2 + y2 = xy $$\frac{d x}{d y}$$

Question 3.
Obtain the differential equation by eliminating the arbitrary constants from the following equations:
(i) y2 = a(b – x)(b + x)
Solution:
y2 = a(b – x)(b + x) = a(b2 – x2)
Differentiating both sides w.r.t. x, we get
2y $$\frac{d y}{d x}$$ = a(0 – 2x) = -2ax
∴ y $$\frac{d y}{d x}$$ = -ax …….(1)
Differentiating again w.r.t. x, we get

This is the required D.E.

(ii) y = a sin(x + b)
Solution:
y = a sin(x + b)

This is the required D.E.

(iii) (y – a)2 = b(x + 4)
Solution:
(y – a)2 = b(x + 4) …….(1)
Differentiating both sides w.r.t. x, we get
$$2(y-a) \cdot \frac{d}{d x}(y-a)=b \frac{d}{d x}(x+4)$$

(iv) y = $$\sqrt{a \cos (\log x)+b \sin (\log x)}$$
Solution:
y = $$\sqrt{a \cos (\log x)+b \sin (\log x)}$$
∴ y2 = a cos (log x) + b sin (log x) …….(1)
Differentiating both sides w.r.t. x, we get

(v) y = Ae3x+1 + Be-3x+1
Solution:
y = Ae3x+1 + Be-3x+1 …… (1)
Differentiating twice w.r.t. x, we get

This is the required D.E.

Question 4.
Form the differential equation of:
(i) all circles which pass through the origin and whose centres lie on X-axis.
Solution:

Let C (h, 0) be the centre of the circle which pass through the origin. Then radius of the circle is h.
∴ equation of the circle is (x – h)2 + (y – 0)2 = h2
∴ x2 – 2hx + h2 + y2 = h2
∴ x2 + y2 = 2hx ……..(1)
Differentiating both sides w.r.t. x, we get
2x + 2y $$\frac{d y}{d x}$$ = 2h
Substituting the value of 2h in equation (1), we get
x2 + y2 = (2x + 2y $$\frac{d y}{d x}$$) x
∴ x2 + y2 = 2x2 + 2xy $$\frac{d y}{d x}$$
∴ 2xy $$\frac{d y}{d x}$$ + x2 – y2 = 0
This is the required D.E.

(ii) all parabolas which have 4b as latus rectum and whose axis is parallel to Y-axis.
Solution:
Let A(h, k) be the vertex of the parabola which has 4b as latus rectum and whose axis is parallel to the Y-axis.
Then equation of the parabola is
(x – h)2 = 4b(y – k) ……. (1)
where h and k are arbitrary constants.

Differentiating both sides of (1) w.r.t. x, we get
2(x – h). $$\frac{d}{d x}$$(x – h) = 4b . $$\frac{d}{d x}$$(y – k)
∴ 2(x – h) x (1 – 0) = 4b($$\frac{d y}{d x}$$ – 0)
∴ (x – h) = 2b $$\frac{d y}{d x}$$
Differentiating again w.r.t. x, we get
1 – 0 = 2b $$\frac{d^{2} y}{d x^{2}}$$
∴ 2b $$\frac{d^{2} y}{d x^{2}}$$ – 1 = 0
This is the required D.E.

(iii) an ellipse whose major axis is twice its minor axis.
Solution:
Let 2a and 2b be lengths of the major axis and minor axis of the ellipse.
Then 2a = 2(2b)
∴ a = 2b
∴ equation of the ellipse is
$$\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$$
∴ $$\frac{x^{2}}{(2 b)^{2}}+\frac{y^{2}}{b^{2}}=1$$
∴ $$\frac{x^{2}}{4 b^{2}}+\frac{y^{2}}{b^{2}}=1$$
∴ x2 + 4y2 = 4b2
Differentiating w.r.t. x, we get
2x + 4 × 2y $$\frac{d y}{d x}$$ = 0
∴ x + 4y $$\frac{d y}{d x}$$ = 0
This is the required D.E.

(iv) all the lines which are normal to the line 3x + 2y + 7 = 0.
Solution:
Slope of the line 3x – 2y + 7 = 0 is $$\frac{-3}{-2}=\frac{3}{2}$$.
∴ slope of normal to this line is $$-\frac{2}{3}$$
Then the equation of the normal is
y = $$-\frac{2}{3}$$x + k, where k is an arbitrary constant.
Differentiating w.r.t. x, we get
$$\frac{d y}{d x}=-\frac{2}{3} \times 1+0$$
∴ 3$$\frac{d y}{d x}$$ + 2 = 0
This is the required D.E.

(v) the hyperbola whose length of transverse and conjugate axes are half of that of the given hyperbola $$\frac{x^{2}}{16}-\frac{y^{2}}{36}=k$$.
Solution:
The equation of the hyperbola is $$\frac{x^{2}}{16}-\frac{y^{2}}{36}=k$$
i.e., $$\frac{x^{2}}{16 k}-\frac{y^{2}}{36 k}=1$$
Comparing this equation with $$\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$$, we get
a2 = 16k, b2 = 36k
∴ a = 4√k, b = 6√k
∴ l(transverse axis) = 2a = 8√k
and l(conjugate axis) = 2b = 12√k
Let 2A and 2B be the lengths of the transverse and conjugate axes of the required hyperbola.
Then according to the given condition
2A = a = 4√k and 2B = b = 6√k
∴ A = 2√k and B = 3√k
∴ equation of the required hyperbola is
$$\frac{x^{2}}{A^{2}}-\frac{y^{2}}{B^{2}}=1$$
i.e., $$\frac{x^{2}}{4 k}-\frac{y^{2}}{9 k}=1$$
∴ 9x2 – 4y2 = 36k, where k is an arbitrary constant.
Differentiating w.r.t. x, we get
9 × 2x – 4 × 2y $$\frac{d y}{d x}$$ = 0
∴ 9x – 4y $$\frac{d y}{d x}$$ = 0
This is the required D.E.

Question 5.
Solve the following differential equations:
(i) log($$\frac{d y}{d x}$$) = 2x + 3y
Solution:

(ii) $$\frac{d y}{d x}$$ = x2y + y
Solution:

(iii) $$\frac{d y}{d x}=\frac{2 y-x}{2 y+x}$$
Solution:

(iv) x dy = (x + y + 1) dx
Solution:

(v) $$\frac{d y}{d x}$$ + y cot x = x2 cot x + 2x
Solution:
$$\frac{d y}{d x}$$ + y cot x = x cot x + 2x ……..(1)
This is the linear differential equation of the form
$$\frac{d y}{d x}$$ + Py = Q, where P = cot x and Q = x2 cot x + 2x
∴ I.F. = $$e^{\int P d x}$$
= $$e^{\int \cot x d x}$$
= $$e^{\log (\sin x)}$$
= sin x
∴ the solution of (1) is given by
y(I.F.) = ∫Q . (I.F.) dx + c
∴ y sin x = ∫(x2 cot x + 2x) sin x dx + c
∴ y sinx = ∫(x2 cot x . sin x + 2x sin x) dx + c
∴ y sinx = ∫x2 cos x dx + 2∫x sin x dx + c
∴ y sinx = x2 ∫cos x dx – ∫[$$\frac{d}{d x}\left(x^{2}\right)$$ ∫cos x dx] dx + 2∫x sin x dx + c
∴ y sin x = x2 (sin x) – ∫2x(sin x) dx + 2∫x sin x dx + c
∴ y sin x = x2 sin x – 2∫x sin x dx + 2∫x sin x dx + c
∴ y sin x = x2 sin x + c
∴ y = x2 + c cosec x
This is the general solution.

(vi) y log y = (log y2 – x) $$\frac{d y}{d x}$$
Solution:

(vii) 4 $$\frac{d x}{d y}$$ + 8x = 5e-3y
Solution:

Question 6.
Find the particular solution of the following differential equations:
(i) y(1 + log x) = (log xx) $$\frac{d y}{d x}$$, when y(e) = e2
Solution:

(ii) (x + 2y2) $$\frac{d y}{d x}$$ = y, when x = 2, y = 1
Solution:

This is the general solution.
When x = 2, y = 1, we have
2 = 2(1)2 + c(1)
∴ c = 0
∴ the particular solution is x = 2y2.

(iii) $$\frac{d y}{d x}$$ – 3y cot x = sin 2x, when y($$\frac{\pi}{2}$$) = 2
Solution:
$$\frac{d y}{d x}$$ – 3y cot x = sin 2x
$$\frac{d y}{d x}$$ = (3 cot x) y = sin 2x ……..(1)
This is the linear differential equation of the form

(iv) (x + y) dy + (x – y) dx = 0; when x = 1 = y
Solution:

(v) $$2 e^{\frac{x}{y}} d x+\left(y-2 x e^{\frac{x}{y}}\right) d y=0$$, when y(0) = 1
Solution:

Question 7.
Show that the general solution of defferential equation $$\frac{d y}{d x}+\frac{y^{2}+y+1}{x^{2}+x+1}=0$$ is given by (x + y + 1) = c(1 – x – y – 2xy).
Solution:

Question 8.
The normal lines to a given curve at each point (x, y) on the curve pass through (2, 0). The curve passes through (2, 3). Find the equation of the curve.
Solution:
Let P(x, y) be a point on the curve y = f(x).
Then slope of the normal to the curve is $$-\frac{1}{\left(\frac{d y}{d x}\right)}$$
∴ equation of the normal is

This is the general equation of the curve.
Since, the required curve passed through the point (2, 3), we get
22 + 32 = 4(2) + c
∴ c = 5
∴ equation of the required curve is x2 + y2 = 4x + 5.

Question 9.
The volume of a spherical balloon being inflated changes at a constant rate. If initially its radius is 3 units and after 3 seconds it is 6 units. Find the radius of the balloon after t seconds.
Solution:
Let r be the radius and V be the volume of the spherical balloon at any time t.
Then the rate of change in volume of the spherical balloon is $$\frac{d V}{d t}$$ which is a constant.

Hence, the radius of the spherical balloon after t seconds is $$(63 t+27)^{\frac{1}{3}}$$ units.

Question 10.
A person’s assets start reducing in such a way that the rate of reduction of assets is proportional to the square root of the assets existing at that moment. If the assets at the beginning are ₹ 10 lakhs and they dwindle down to ₹ 10,000 after 2 years, show that the person will be bankrupt in 2$$\frac{2}{9}$$ years from the start.
Solution:
Let x be the assets of the presort at time t years.
Then the rate of reduction is $$\frac{d x}{d t}$$ which is proportional to √x.
∴ $$\frac{d x}{d t}$$ ∝ √x
∴ $$\frac{d x}{d t}$$ = -k√x, where k > 0
∴ $$\frac{d x}{\sqrt{x}}$$ = -k dt
Integrating both sides, we get
$$\int x^{-\frac{1}{2}} d x$$ = -k∫dt
∴ $$\frac{x^{\frac{1}{2}}}{\left(\frac{1}{2}\right)}$$ = -kt + c
∴ 2√x = -kt + c
At the beginning, i.e. at t = 0, x = 10,00,000
2√10,00,000 = -k(0) + c
∴ c = 2000
∴ 2√x = -kt + 2000 ……..(1)
Also, when t = 2, x = 10,000
∴ 2√10000 = -k × 2 + 2000
∴ 2k = 1800
∴ k = 900
∴ (1) becomes,
∴ 2√x = -900t + 2000
When the person will be bankrupt, x = 0
∴ 0 = -900t + 2000
∴ 900t = 2000
∴ t = $$\frac{20}{9}=2 \frac{2}{9}$$
Hence, the person will be bankrupt in $$2 \frac{2}{9}$$ years.

## Maharashtra Board 12th Maths Solutions Chapter 6 Differential Equations Ex 6.6

Balbharti 12th Maharashtra State Board Maths Solutions Book Pdf Chapter 6 Differential Equations Ex 6.6 Questions and Answers.

## Maharashtra State Board 12th Maths Solutions Chapter 6 Differential Equations Ex 6.6

Question 1.
In a certain culture of bacteria, the rate of increase is proportional to the number present. If it is found that the number doubles in 4 hours, find the number of times the bacteria are increased in 12 hours.
Solution:
Let x be the number of bacteria in the culture at time t.
Then the rate of increase is $$\frac{d x}{d t}$$ which is proportional to x.
∴ $$\frac{d x}{d t}$$ ∝ x
∴ $$\frac{d x}{d t}$$ = kx, where k is a constant
∴ $$\frac{d x}{x}$$ = k dt
On integrating, we get
$$\int \frac{d x}{x}$$ = k∫dt + c
∴ log x = kt + c
Initially, i.e. when t = 0, let x = x0
log x0 = k × 0 + c
∴ c = log x0
∴ log x = kt + log x0
∴ log x – log x0 = kt
∴ log($$\frac{x}{x_{0}}$$) = kt ………(1)
Since the number doubles in 4 hours, i.e. when t = 4, x = 2x0

∴ the number of bacteria will be 8 times the original number in 12 hours.

Question 2.
If the population of a country doubles in 60 years; in how many years will it be triple (treble) under the assumption that the rate of increase is proportional to the number of inhabitants?
[Given log 2 = 0.6912, log 3 = 1.0986]
Solution:
Let P be the population at time t years.
Then $$\frac{d P}{d t}$$, the rate of increase of population is proportional to P.
∴ $$\frac{d P}{d t}$$ ∝ P
∴ $$\frac{d P}{d t}$$ = kP, where k is a constant
∴ $$\frac{d P}{P}$$ = k dt
On integrating, we get
$$\int \frac{d P}{P}$$ = k∫dt + c
∴ log P = kt + c
Initially i.e. when t = 0, let P = P0
∴ log P0 = k x 0 + c
∴ c = log P0
∴ log P = kt + log P0
∴ log P – log P0 = kt
∴ log($$\frac{P}{P_{0}}$$) = kt ……(1)
Since, the population doubles in 60 years, i.e. when t = 60, P = 2P0

∴ the population becomes triple in 95.4 years (approximately).

Question 3.
If a body cools from 80°C to 50°C at room temperature of 25°C in 30 minutes, find the temperature of the body after 1 hour.
Solution:
Let θ°C be the temperature of the body at time t minutes.
The room temperature is given to be 25°C.
Then by Newton’s law of cooling, $$\frac{d \theta}{d t}$$, the rate of change of temperature, is proportional to (θ – 25).

∴ the temperature of the body will be 36.36°C after 1 hour.

Question 4.
The rate of growth of bacteria is proportional to the number present. If initially, there were 1000 bacteria and the number doubles in 1 hour, find the number of bacteria after 2½ hours. [Take √2 = 1.414]
Solution:
Let x be the number of bacteria at time t.
Then the rate of increase is $$\frac{d x}{d t}$$ which is proportional to x.
∴ $$\frac{d x}{d t}$$ ∝ x
∴ $$\frac{d x}{d t}$$ = kx, where k is a constant
∴ $$\frac{d x}{x}$$ = k dt
On integrating, we get
$$\int \frac{d x}{x}$$ = k∫dt + c
∴ log x = kt + c
Initially, i.e. when t = 0, x = 1000
∴ log 1000 = k × 0 + c
∴ c = log 1000
∴ log x = kt + log 1000
∴ log x – log 1000 = kt
∴ log($$\frac{x}{1000}$$) = kt ……(1)
Now, when t = 1, x = 2 × 1000 = 2000
∴ log($$\frac{2000}{1000}$$) = k
∴ k = log 2

∴ the number of bacteria after 2½ hours = 5656.

Question 5.
The rate of disintegration of a radioactive element at any time t is proportional to its mass at that time. Find the time during which the original mass of 1.5 gm will disintegrate into its mass of 0.5 gm.
Solution:
Let m be the mass of the radioactive element at time t.
Then the rate of disintegration is $$\frac{d m}{d t}$$ which is proportional to m.

∴ log(3)-1 = -kt
∴ -log 3 = -kt
∴ t = $$\frac{1}{k}$$ log 3
∴ the original mass will disintegrate to 0.5 gm when t = $$\frac{1}{k}$$ log 3

Question 6.
The rate of decay of certain substances is directly proportional to the amount present at that instant. Initially, there is 25 gm of certain substance and two hours later it is found that 9 gm are left. Find the amount left after one more hour.
Solution:
Let x gm be the amount of the substance left at time t.
Then the rate of decay is $$\frac{d x}{d t}$$, which is proportional to x.

∴ $$\frac{x}{25}=\frac{27}{125}$$
∴ x = $$\frac{27}{5}$$
∴ the amount left after 3 hours $$\frac{27}{5}$$ gm.

Question 7.
Find the population of a city at any time t, given that the rate of increase of population is proportional to the population at that instant and that in a period of 40 years, the population increased from 30,000 to 40,000.
Solution:
Let P be the population of the city at time t.
Then $$\frac{d P}{d t}$$, the rate of increase of population is proportional to P.
∴ $$\frac{d P}{d t}$$ ∝ P
∴ $$\frac{d P}{d t}$$ = kP, where k is a constant.
∴ $$\frac{d P}{P}$$ = k dt
On integrating, we get
$$\int \frac{1}{P} d P$$ = k∫dt + c
∴ log P = kt + c
Initially, i.e. when t = 0, P = 30000
∴ log 30000 = k × 0 + c
∴ c = log 30000
∴ log P = kt + log 30000
∴ log P – log 30000 = kt
∴ log($$\frac{P}{30000}$$) = kt …….(1)
Now, when t = 40, P = 40000

∴ the population of the city at time t = 30000$$\left(\frac{4}{3}\right)^{\frac{t}{40}}$$.

Question 8.
A body cools according to Newton’s law from 100°C to 60°C in 20 minutes. The temperature of the surroundings is 20°C. How long will it take to cool down to 30°C?
Solution:
Let θ°C be the temperature of the body at time t.
The temperature of the surrounding is given to be 20°C.
According to Newton’s law of cooling

∴ the body will cool down to 30°C in 60 minutes, i.e. in 1 hour.

Question 9.
A right circular cone has a height of 9 cm and a radius of the base of 5 cm. It is inverted and water is poured into it. If at any instant the water level rises at the rate of $$\left(\frac{\pi}{A}\right)$$ cm/sec, where A is the area of the water surface
at that instant, show that the vessel will be full in 75 seconds.
Solution:

Let r be the radius of the water surface and h be the height of the water at time t.
∴ area of the water surface A = πr2 sq cm.
Since height of the right circular cone is 9 cm and radius of the base is 5 cm.
$$\frac{r}{h}=\frac{5}{9}$$
∴ r = $$\frac{5}{9} h$$
∴ area of water surface, i.e. A = $$\pi\left(\frac{5}{9} h\right)^{2}$$
∴ A = $$\frac{25 \pi h^{2}}{81}$$ ……..(1)
The water level, i.e. the rate of change of h is $$\frac{d h}{d t}$$ rises at the rate of $$\left(\frac{\pi}{A}\right)$$ cm/sec.

∴ t = $$\frac{81 \times 9 \times 25}{3 \times 81}$$ = 75
Hence, the vessel will be full in 75 seconds.

Question 10.
Assume that a spherical raindrop evaporates at a rate proportional to its surface area. If its radius originally is 3 mm and 1 hour later has been reduced to 2 mm, find an expression for the radius of the raindrop at any time t.
Solution:
Let r be the radius, V be the volume and S be the surface area of the spherical raindrop at time t.
Then V = $$\frac{4}{3}$$πr3 and S = 4πr2
The rate at which the raindrop evaporates is $$\frac{d V}{d t}$$ which is proportional to the surface area.
∴ $$\frac{d V}{d t}$$ ∝ S
∴ $$\frac{d V}{d t}$$ = -kS, where k > 0 ………(1)

On integrating, we get
∫dr = -k∫dt + c
∴ r = -kt + c
Initially, i.e. when t = 0, r = 3
∴ 3 = -k × 0 + c
∴ c = 3
∴ r = -kt + 3
When t = 1, r = 2
∴ 2 = -k × 1 + 3
∴ k = 1
∴ r = -t + 3
∴ r = 3 – t, where 0 ≤ t ≤ 3.
This is the required expression for the radius of the raindrop at any time t.

Question 11.
The rate of growth of the population of a city at any time t is proportional to the size of the population. For a certain city, it is found that the constant of proportionality is 0.04. Find the population of the city after 25 years, if the initial population is 10,000. [Take e = 2.7182]
Solution:
Let P be the population of the city at time t.
Then the rate of growth of population is $$\frac{d P}{d t}$$ which is proportional to P.
∴ $$\frac{d P}{d t}$$ ∝ P
∴ $$\frac{d P}{d t}$$ = kP, where k = 0.04
∴ $$\frac{d P}{d t}$$ = (0.04)P
∴ $$\frac{1}{P}$$ dP = (0.04)dt
On integrating, we get
$$\int \frac{1}{P} d P$$ = (0.04) ∫dt + c
∴ log P = (0.04)t + c
Initially, i.e., when t = 0, P = 10000
∴ log 10000 = (0.04) × 0 + c
∴ c = log 10000
∴ log P = (0.04)t + log 10000
∴ log P – log 10000 = (0.04)t
∴ log($$\frac{P}{10000}$$) = (0.04)t
When t = 25, then
∴ log($$\frac{P}{10000}$$) = 0.04 × 25 = 1
∴ log($$\frac{P}{10000}$$) = log e ……[∵ log e = 1]
∴ $$\frac{P}{10000}$$ = e = 2.7182
∴ P = 2.7182 × 10000 = 27182
∴ the population of the city after 25 years will be 27,182.

Question 12.
Radium decomposes at a rate proportional to the amount present at any time. If p percent of the amount disappears in one year, what percent of the amount of radium will be left after 2 years?
Solution:
Let x be the amount of the radium at time t.
Then the rate of decomposition is $$\frac{d x}{d t}$$ which is proportional to x.

Hence, $$\left(10-\frac{p}{10}\right)^{2} \%$$ of the amount will be left after 2 years.

## Maharashtra Board 12th Maths Solutions Chapter 6 Differential Equations Ex 6.5

Balbharti 12th Maharashtra State Board Maths Solutions Book Pdf Chapter 6 Differential Equations Ex 6.5 Questions and Answers.

## Maharashtra State Board 12th Maths Solutions Chapter 6 Differential Equations Ex 6.5

Question 1.
Solve the following differential equations:
(i) $$\frac{d y}{d x}+\frac{y}{x}=x^{3}-3$$
Solution:
$$\frac{d y}{d x}+\frac{y}{x}=x^{3}-3$$ …….(1)
This is the linear differential equation of the form
$$\frac{d y}{d x}$$ + P . y = Q, where P = $$\frac{1}{x}$$ and Q = x3 – 3

This is the general solution.

(ii) cos2x . $$\frac{d y}{d x}$$ + y = tan x
Solution:

(iii) (x + 2y3) $$\frac{d y}{d x}$$ = y
Solution:

(iv) $$\frac{d y}{d x}$$ + y . sec x = tan x
Solution:
$$\frac{d y}{d x}$$ + y sec x = tan x
∴ $$\frac{d y}{d x}$$ + (sec x) . y = tan x ……..(1)
This is the linear differential equation of the form
$$\frac{d y}{d x}$$ + P . y = Q, where P = sec x and Q = tan x
∴ I.F. = $$e^{\int P d x}$$
= $$e^{\int \sec x d x}$$
= $$e^{\log (\sec x+\tan x)}$$
= sec x + tan x
∴ the solution of (1) is given by
y (I.F.) = ∫Q . (I.F.) dx + c
∴ y(sec x + tan x) = ∫tan x (sec x + tan x) dx + c
∴ (sec x + tan x) . y = ∫(sec x tan x + tan2x) dx + c
∴ (sec x + tan x) . y = ∫(sec x tan x + sec2x – 1) dx + c
∴ (sec x + tan x) . y = sec x + tan x – x + c
∴ y(sec x + tan x) = sec x + tan x – x + c
This is the general solution.

(v) x $$\frac{d y}{d x}$$ + 2y = x2 . log x
Solution:

(vi) (x + y) $$\frac{d y}{d x}$$ = 1
Solution:
(x + y) $$\frac{d y}{d x}$$ = 1
∴ $$\frac{d x}{d y}$$ = x + y
∴ $$\frac{d x}{d y}$$ – x = y
∴ $$\frac{d x}{d y}$$ + (-1) x = y ……….(1)
This is the linear differential equation of the form

This is the general solution.

(vii) (x + a) $$\frac{d y}{d x}$$ – 3y = (x + a)5
Solution:

(viii) dr + (2r cot θ + sin 2θ) dθ = 0
Solution:
dr + (2r cot θ + sin 2θ) dθ = 0
∴ $$\frac{d r}{d \theta}$$ + (2r cot θ + sin 2θ) = 0
∴ $$\frac{d r}{d \theta}$$ + (2 cot θ)r = -sin 2θ ………(1)
This is the linear differential equation of the form dr
$$\frac{d r}{d \theta}$$ + P . r = Q, where P = 2 cot θ and Q = -sin 2θ

This is the general solution.

(ix) y dx + (x – y2) dy = 0
Solution:
y dx + (x – y2) dy = 0
∴ y dx = -(x – y2) dy
∴ $$\frac{d x}{d y}=-\frac{\left(x-y^{2}\right)}{y}=-\frac{x}{y}+y$$
∴ $$\frac{d x}{d y}+\left(\frac{1}{y}\right) \cdot x=y$$ ………(1)
This is the linear differential equation of the form

This is the general solution.

(x) $$\left(1-x^{2}\right) \frac{d y}{d x}+2 x y=x\left(1-x^{2}\right)^{\frac{1}{2}}$$
Solution:

(xi) $$\left(1+x^{2}\right) \frac{d y}{d x}+y=e^{\tan ^{-1} x}$$
Solution:

Question 2.
Find the equation of the curve which passes through the origin and has the slope x + 3y – 1 at any point (x, y) on it.
Solution:
Let A(x, y) be the point on the curve y = f(x).
Then slope of the tangent to the curve at the point A is $$\frac{d y}{d x}$$.
According to the given condition,
$$\frac{d y}{d x}$$ = x + 3y – 1
∴ $$\frac{d y}{d x}$$ – 3y = x – 1 ………(1)
This is the linear differential equation of the form

This is the general equation of the curve.
But the required curve is passing through the origin (0, 0).
∴ by putting x = 0 and y = 0 in (2), we get
0 = 2 + c
∴ c = -2
∴ from (2), the equation of the required curve is 3(x + 3y) = 2 – 2e3x i.e. 3(x + 3y) = 2 (1 – e3x).

Question 3.
Find the equation of the curve passing through the point $$\left(\frac{3}{\sqrt{2}}, \sqrt{2}\right)$$ having slope of the tangent to the curve at any point (x, y) is $$-\frac{4 x}{9 y}$$.
Solution:
Let A(x, y) be the point on the curve y = f(x).
Then the slope of the tangent to the curve at point A is $$\frac{d y}{d x}$$.
According to the given condition
$$\frac{d y}{d x}=-\frac{4 x}{9 y}$$
∴ y dy = $$-\frac{4}{9}$$ x dx
Integrating both sides, we get
∫y dy= $$-\frac{4}{9}$$ ∫x dx
∴ $$\frac{y^{2}}{2}=-\frac{4}{9} \cdot \frac{x^{2}}{2}+c_{1}$$
∴ 9y2 = -4x2 + 18c1
∴ 4x2 + 9y2 = c where c = 18c1
This is the general equation of the curve.
But the required curve is passing through the point $$\left(\frac{3}{\sqrt{2}}, \sqrt{2}\right)$$.
∴ by putting x = $$\frac{3}{\sqrt{2}}$$ and y = √2 in (1), we get
$$4\left(\frac{3}{\sqrt{2}}\right)^{2}+9(\sqrt{2})^{2}=c$$
∴ 18 + 18 = c
∴ c = 36
∴ from (1), the equation of the required curve is 4x2 + 9y2 = 36.

Question 4.
The curve passes through the point (0, 2). The sum of the coordinates of any point on the curve exceeds the slope of the tangent to the curve at any point by 5. Find the equation of the curve.
Solution:
Let A(x, y) be any point on the curve.
Then slope of the tangent to the curve at the point A is $$\frac{d y}{d x}$$.
According to the given condition
x + y = $$\frac{d y}{d x}$$ + 5
∴ $$\frac{d y}{d x}$$ – y = x – 5 ………(1)
This is the linear differential equation of the form
$$\frac{d y}{d x}$$ + P . y = Q, where P = -1 and Q = x – 5
∴ I.F. = $$e^{\int P d x}=e^{\int-1 d x}=e^{-x}$$
∴ the solution of (1) is given by

This is the general equation of the curve.
But the required curve is passing through the point (0, 2).
∴ by putting x = 0, y = 2 in (2), we get
2 = 4 – 0 + c
∴ c = -2
∴ from (2), the equation of the required curve is y = 4 – x – 2ex.

Question 5.
If the slope of the tangent to the curve at each of its point is equal to the sum of abscissa and the product of the abscissa and ordinate of the point. Also, the curve passes through the point (0, 1). Find the equation of the curve.
Solution:
Let A(x, y) be the point on the curve y = f(x).
Then slope of the tangent to the curve at the point A is $$\frac{d y}{d x}$$.
According to the given condition
$$\frac{d y}{d x}$$ = x + xy
∴ $$\frac{d y}{d x}$$ – xy = x ……….. (1)
This is the linear differential equation of the form
$$\frac{d y}{d x}$$ + Py = Q, where P = -x and Q = x
∴ I.F. = $$e^{\int P d x}=e^{\int-x d x}=e^{-\frac{x^{2}}{2}}$$
∴ the solution of (1) is given by
y . (I.F.) = ∫Q . (I.F.) dx + c

This is the general equation of the curve.
But the required curve is passing through the point (0, 1).
∴ by putting x = 0 and y = 1 in (2), we get
1 + 1 = c
∴ c = 2
∴ from (2), the equation of the required curve is 1 + y = $$2 e^{\frac{x^{2}}{2}}$$.

## Maharashtra Board 12th Maths Solutions Chapter 6 Differential Equations Ex 6.4

Balbharti 12th Maharashtra State Board Maths Solutions Book Pdf Chapter 6 Differential Equations Ex 6.4 Questions and Answers.

## Maharashtra State Board 12th Maths Solutions Chapter 6 Differential Equations Ex 6.4

I. Solve the following differential equations:

Question 1.
$$x \sin \left(\frac{y}{x}\right) d y=\left[y \sin \left(\frac{y}{x}\right)-x\right] d x$$
Solution:

Question 2.
(x2 + y2) dx – 2xy . dy = 0
Solution:
(x2 + y2) dx – 2xy dy = 0
∴ 2xy dy = (x2 + y2) dx
∴ $$\frac{d y}{d x}=\frac{x^{2}+y^{2}}{2 x y}$$ ………(1)

Question 3.
$$\left(1+2 e^{\frac{x}{y}}\right)+2 e^{\frac{x}{y}}\left(1-\frac{x}{y}\right) \frac{d y}{d x}=0$$
Solution:

Question 4.
y2 dx + (xy + x2) dy = 0
Solution:
y2 dx + (xy + x2) dy = 0
∴ (xy + x2) dy = -y2 dx
∴ $$\frac{d y}{d x}=\frac{-y^{2}}{x y+x^{2}}$$ ……..(1)
Put y = vx
∴ $$\frac{d y}{d x}=v+x \frac{d v}{d x}$$
Substituting these values in (1), we get

Question 5.
(x2 – y2) dx + 2xy dy = 0
Solution:

Question 6.
$$\frac{d y}{d x}+\frac{x-2 y}{2 x-y}=0$$
Solution:

Question 7.
$$x \frac{d y}{d x}-y+x \sin \left(\frac{y}{x}\right)=0$$
Solution:

Question 8.
$$\left(1+e^{\frac{x}{y}}\right) d x+e^{\frac{x}{y}}\left(1-\frac{X}{y}\right) d y=0$$
Solution:

Question 9.
$$y^{2}-x^{2} \frac{d y}{d x}=x y \frac{d y}{d x}$$
Solution:

Question 10.
xy $$\frac{d y}{d x}$$ = x2 + 2y2, y(1) = 0
Solution:

Question 11.
x dy + 2y · dx = 0, when x = 2, y = 1
Solution:
∴ x dy + 2y · dx = 0
∴ x dy = -2y dx
∴ $$\frac{1}{y} d y=\frac{-2}{x} d x$$
Integrating, we get

This is the general solution.
When x = 2, y = 1, we get
4(1) = c
∴ c = 4
∴ the particular solution is x2y = 4.

Question 12.
x2 $$\frac{d y}{d x}$$ = x2 + xy + y2
Solution:
x2 $$\frac{d y}{d x}$$ = x2 + xy + y2
∴ $$\frac{d y}{d x}=\frac{x^{2}+x y+y^{2}}{x^{2}}$$ ………(1)
Put y = vx
∴ $$\frac{d y}{d x}=v+x \frac{d v}{d x}$$

Question 13.
(9x + 5y) dy + (15x + 11y) dx = 0
Solution:
(9x + 5y) dy + (15x + 11y) dx = 0
∴ (9x + 5y) dy = -(15x + 11y) dx
∴ $$\frac{d y}{d x}=\frac{-(15 x+11 y)}{9 x+5 y}$$ ………(1)
Put y = vx
∴ $$\frac{d y}{d x}=v+x \frac{d v}{d x}$$

Question 14.
(x2 + 3xy + y2) dx – x2 dy = 0
Solution:
(x2 + 3xy + y2) dx – x2 dy = 0
∴ x2 dy = (x2 + 3xy + y2) dx
∴ $$\frac{d y}{d x}=\frac{x^{2}+3 x y+y^{2}}{x^{2}}$$ ………(1)

Question 15.
(x2 + y2) dx – 2xy dy = 0.
Solution:

## Maharashtra Board 12th Maths Solutions Chapter 6 Differential Equations Ex 6.3

Balbharti 12th Maharashtra State Board Maths Solutions Book Pdf Chapter 6 Differential Equations Ex 6.3 Questions and Answers.

## Maharashtra State Board 12th Maths Solutions Chapter 6 Differential Equations Ex 6.3

Question 1.
In each of the following examples verify that the given expression is a solution of the corresponding differential equation.
(i) xy = log y + c; $$\frac{d y}{d x}=\frac{y^{2}}{1-x y}$$
Solution:
xy = log y + c
Differentiating w.r.t. x, we get

Hence, xy = log y + c is a solution of the D.E.
$$\frac{d y}{d x}=\frac{y^{2}}{1-x y^{\prime}}, x y \neq 1$$

(ii) y = (sin-1x)2 + c; (1 – x2) $$\frac{d^{2} y}{d x^{2}}-x \frac{d y}{d x}=2$$
Solution:
y = (sin-1 x)2 + c …….(1)
Differentiating w.r.t. x, we get

Differentiating again w.r.t. x, we get

(iii) y = e-x + Ax + B; $$e^{x} \frac{d^{2} y}{d x^{2}}=1$$
Solution:
y = e-x + Ax + B
Differentiating w.r.t. x, we get

∴ $$e^{x} \frac{d^{2} y}{d x^{2}}=1$$
Hence, y = e-x + Ax + B is a solution of the D.E.
$$e^{x} \frac{d^{2} y}{d x^{2}}=1$$

(iv) y = xm; $$x^{2} \frac{d^{2} y}{d x^{2}}-m x \frac{d y}{d x}+m y=0$$
Solution:
y = xm
Differentiating twice w.r.t. x, we get

This shows that y = xm is a solution of the D.E.
$$x^{2} \frac{d^{2} y}{d x^{2}}-m x \frac{d y}{d x}+m y=0$$

(v) y = a + $$\frac{b}{x}$$; $$x \frac{d^{2} y}{d x^{2}}+2 \frac{d y}{d x}=0$$
Solution:
y = a + $$\frac{b}{x}$$
Differentiating w.r.t. x, we get

Differentiating again w.r.t. x, we get

Hence, y = a + $$\frac{b}{x}$$ is a solution of the D.E.
$$x \frac{d^{2} y}{d x^{2}}+2 \frac{d y}{d x}=0$$

(vi) y = eax; x $$\frac{d y}{d x}$$ = y log y
Solution:
y = eax
log y = log eax = ax log e
log y = ax …….(1) ……..[∵ log e = 1]
Differentiating w.r.t. x, we get
$$\frac{1}{y} \cdot \frac{d y}{d x}$$ = a × 1
∴ $$\frac{d y}{d x}$$ = ay
∴ x $$\frac{d y}{d x}$$ = (ax)y
∴ x $$\frac{d y}{d x}$$ = y log y ………[By (1)]
Hence, y = eax is a solution of the D.E.
x $$\frac{d y}{d x}$$ = y log y.

Question 2.
Solve the following differential equations.
(i) $$\frac{d y}{d x}=\frac{1+y^{2}}{1+x^{2}}$$
Solution:

(ii) log($$\frac{d y}{d x}$$) = 2x + 3y
Solution:

(iii) y – x $$\frac{d y}{d x}$$ = 0
Solution:
y – x $$\frac{d y}{d x}$$ = 0
∴ x $$\frac{d y}{d x}$$ = y
∴ $$\frac{1}{x} d x=\frac{1}{y} d y$$
Integrating both sides, we get
$$\int \frac{1}{x} d x=\int \frac{1}{y} d y$$
∴ log |x| = log |y| + log c
∴ log |x| = log |cy|
∴ x = cy
This is the general solution.

(iv) sec2x . tan y dx + sec2y . tan x dy = 0
Solution:
sec2x . tan y dx + sec2y . tan x dy = 0
∴ $$\frac{\sec ^{2} x}{\tan x} d x+\frac{\sec ^{2} y}{\tan y} d y=0$$
Integrating both sides, we get
$$\int \frac{\sec ^{2} x}{\tan x} d x+\int \frac{\sec ^{2} y}{\tan y} d y=c_{1}$$
Each of these integrals is of the type
$$\int \frac{f^{\prime}(x)}{f(x)} d x$$ = log |f(x)| + c
∴ the general solution is
∴ log|tan x| + log|tan y | = log c, where c1 = log c
∴ log |tan x . tan y| = log c
∴ tan x . tan y = c
This is the general solution.

(v) cos x . cos y dy – sin x . sin y dx = 0
Solution:
cos x . cos y dy – sin x . sin y dx = 0
$$\frac{\cos y}{\sin y} d y-\frac{\sin x}{\cos x} d x=0$$
Integrating both sides, we get
∫cot y dy – ∫tan x dx = c1
∴ log|sin y| – [-log|cos x|] = log c, where c1 = log c
∴ log |sin y| + log|cos x| = log c
∴ log|sin y . cos x| = log c
∴ sin y . cos x = c
This is the general solution.

(vi) $$\frac{d y}{d x}$$ = -k, where k is a constant.
Solution:
$$\frac{d y}{d x}$$ = -k
∴ dy = -k dx
Integrating both sides, we get
∫dy = -k∫dx
∴ y = -kx + c
This is the general solution.

(vii) $$\frac{\cos ^{2} y}{x} d y+\frac{\cos ^{2} x}{y} d x=0$$
Solution:
$$\frac{\cos ^{2} y}{x} d y+\frac{\cos ^{2} x}{y} d x=0$$
∴ y cos2y dy + x cos2x dx = 0
∴ $$x\left(\frac{1+\cos 2 x}{2}\right) d x+y\left(1+\frac{\cos 2 y}{2}\right) d y=0$$
∴ x(1 + cos 2x) dx + y(1 + cos 2y) dy = 0
∴ x dx + x cos 2x dx + y dy+ y cos 2y dy = 0
Integrating both sides, we get
∫x dx + ∫y dy + ∫x cos 2x dx + ∫y cos 2y dy = c1 ……..(1)
Using integration by parts

Multiplying throughout by 4, this becomes
2x2 + 2y2 + 2x sin 2x + cos 2x + 2y sin 2y + cos 2y = 4c1
∴ 2(x2 + y2) + 2(x sin 2x + y sin 2y) + cos 2y + cos 2x + c = 0, where c = -4c1
This is the general solution.

(viii) $$y^{3}-\frac{d y}{d x}=x^{2} \frac{d y}{d x}$$
Solution:

(ix) 2ex+2y dx – 3 dy = 0
Solution:

(x) $$\frac{d y}{d x}$$ = ex+y + x2 ey
Solution:

∴ 3ex + 3e-y + x3 = -3c1
∴ 3ex + 3e-y + x3 = c, where c = -3c1
This is the general solution.

Question 3.
For each of the following differential equations, find the particular solution satisfying the given condition:
(i) 3ex tan y dx + (1 + ex) sec2y dy = 0, when x = 0, y = π
Solution:
3ex tan y dx + (1 + ex) sec2y dy = 0

(ii) (x – y2x) dx – (y + x2y) dy = 0, when x = 2, y = 0
Solution:
(x – y2x) dx – (y + x2y) dy = 0
∴ x(1 – y2) dx – y(1 + x2) dy = 0

When x = 2, y = 0, we have
(1 + 4)(1 – 0) = c
∴ c = 5
∴ the particular solution is (1 + x2)(1 – y2) = 5.

(iii) y(1 + log x) $$\frac{d x}{d y}$$ – x log x = 0, y = e2, when x = e
Solution:
y(1 + log x) $$\frac{d x}{d y}$$ – x log x = 0

(iv) (ey + 1) cos x + ey sin x $$\frac{d y}{d x}$$ = 0, when x = $$\frac{\pi}{6}$$, y = 0
Solution:
(ey + 1) cos x + ey sin x $$\frac{d y}{d x}$$ = 0

$$\int \frac{f^{\prime}(x)}{f(x)} d x$$ = log|f(x)| + c
∴ from (1), the general solution is
log|sin x| + log|ey + 1| = log c, where c1 = log c
∴ log|sin x . (ey + 1)| = log c
∴ sin x . (ey + 1) = c
When x = $$\frac{\pi}{4}$$, y = 0, we get
$$\left(\sin \frac{\pi}{4}\right)\left(e^{0}+1\right)=c$$
∴ c = $$\frac{1}{\sqrt{2}}$$(1 + 1) = √2
∴ the particular solution is sin x . (ey + 1) = √2

(v) (x + 1) $$\frac{d y}{d x}$$ – 1 = 2e-y, y = 0, when x = 1
Solution:

This is the general solution.
Now, y = 0, when x = 1
∴ 2 + e0 = c(1 + 1)
∴ 3 = 2c
∴ c = $$\frac{3}{2}$$
∴ the particular solution is 2 + ey = $$\frac{3}{2}$$ (x + 1)
∴ 2(2 + ey) = 3(x + 1).

(vi) cos($$\frac{d y}{d x}$$) = a, a ∈ R, y (0) = 2
Solution:
cos($$\frac{d y}{d x}$$) = a
∴ $$\frac{d y}{d x}$$ = cos-1 a
∴ dy = (cos-1 a) dx
Integrating both sides, we get
∫dy = (cos-1 a) ∫dx
∴ y = (cos-1 a) x + c
∴ y = x cos-1 a + c
This is the general solution.
Now, y(0) = 2, i.e. y = 2,
when x = 0, 2 = 0 + c
∴ c = 2
∴ the particular solution is
∴ y = x cos-1 a + 2
∴ y – 2 = x cos-1 a
∴ $$\frac{y-2}{x}$$ = cos-1a
∴ cos($$\frac{y-2}{x}$$) = a

Question 4.
Reduce each of the following differential equations to the variable separable form and hence solve:
(i) $$\frac{d y}{d x}$$ = cos(x + y)
Solution:

(ii) (x – y)2 $$\frac{d y}{d x}$$ = a2
Solution:

(iii) x + y $$\frac{d y}{d x}$$ = sec(x2 + y2)
Solution:

Integrating both sides, we get
∫cos u du = 2 ∫dx
∴ sin u = 2x + c
∴ sin(x2 + y2) = 2x + c
This is the general solution.

(iv) cos2(x – 2y) = 1 – 2 $$\frac{d y}{d x}$$
Solution:

Integrating both sides, we get
∫dx = ∫sec2u du
∴ x = tan u + c
∴ x = tan(x – 2y) + c
This is the general solution.

(v) (2x – 2y + 3) dx – (x – y + 1) dy = 0, when x = 0, y = 1
Solution:
(2x – 2y + 3) dx – (x – y + 1) dy = 0
∴ (x – y + 1) dy = (2x – 2y + 3) dx
∴ $$\frac{d y}{d x}=\frac{2(x-y)+3}{(x-y)+1}$$ ………(1)
Put x – y = u, Then $$1-\frac{d y}{d x}=\frac{d u}{d x}$$

∴ u – log|u + 2| = -x + c
∴ x – y – log|x – y + 2| = -x + c
∴ (2x – y) – log|x – y + 2| = c
This is the general solution.
Now, y = 1, when x = 0.
∴ (0 – 1) – log|0 – 1 + 2| = c
∴ -1 – o = c
∴ c = -1
∴ the particular solution is
(2x – y) – log|x – y + 2| = -1
∴ (2x – y) – log|x – y + 2| + 1 = 0