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NCERT Solutions Class 12 Mathematics Part III . 627 Pages (1343-1970). Free Flip-Book by Study Innovations

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NCERT Solutions Class 12 Mathematics Part III . 627 Pages (1343-1970). Free Flip-Book by Study Innovations

NCERT Solutions Class 12 Mathematics Part III . 627 Pages (1343-1970). Free Flip-Book by Study Innovations

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Class XII Chapter 9 – Differential Equations Maths

Answer
The differential equation of the given curve is:

Integrating both sides, we get:

Now, the curve passes through point (1, –1).

Substituting C = –2 in equation (1), we get:

This is the required solution of the given curve.
Question 17:
Find the equation of a curve passing through the point (0, –2) given that at any point

on the curve, the product of the slope of its tangent and y-coordinate of the point
is equal to the x-coordinate of the point.
Answer
Let x and y be the x-coordinate and y-coordinate of the curve respectively.

Page 43 of 120

Class XII Chapter 9 – Differential Equations Maths

We know that the slope of a tangent to the curve in the coordinate axis is given by the

According to the given information, we get:

Integrating both sides, we get:

Now, the curve passes through point (0, –2).

∴ (–2)2 – 02 = 2C

⇒ 2C = 4

Substituting 2C = 4 in equation (1), we get:
y2 – x2 = 4
This is the required equation of the curve.
Question 18:
At any point (x, y) of a curve, the slope of the tangent is twice the slope of the line
segment joining the point of contact to the point (–4, –3). Find the equation of the curve
given that it passes through (–2, 1).
Answer

Page 44 of 120

Class XII Chapter 9 – Differential Equations Maths

It is given that (x, y) is the point of contact of the curve and its tangent.

The slope (m1) of the line segment joining (x, y) and (–4, –3) is
We know that the slope of the tangent to the curve is given by the relation,

According to the given information:

Integrating both sides, we get:

This is the general equation of the curve.
It is given that it passes through point (–2, 1).

Substituting C = 1 in equation (1), we get:
y + 3 = (x + 4)2
This is the required equation of the curve.

Page 45 of 120

Class XII Chapter 9 – Differential Equations Maths

Question 19:
The volume of 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 balloon after t
seconds.
Answer
Let the rate of change of the volume of the balloon be k (where k is a constant).

Integrating both sides, we get:

⇒ 4π × 33 = 3 (k × 0 + C)

⇒ 108π = 3C

⇒ C = 36π

Page 46 of 120

Class XII Chapter 9 – Differential Equations Maths
At t = 3, r = 6:

⇒ 4π × 63 = 3 (k × 3 + C)

⇒ 864π = 3 (3k + 36π)

⇒ 3k = –288π – 36π = 252π

⇒ k = 84π
Substituting the values of k and C in equation (1), we get:

Thus, the radius of the balloon after t seconds is .

Question 20:
In a bank, principal increases continuously at the rate of r% per year. Find the value of r
if Rs 100 doubles itself in 10 years (loge 2 = 0.6931).

Page 47 of 120

Class XII Chapter 9 – Differential Equations Maths

Answer
Let p, t, and r represent the principal, time, and rate of interest respectively.
It is given that the principal increases continuously at the rate of r% per year.

Integrating both sides, we get:

It is given that when t = 0, p = 100.
⇒ 100 = ek … (2)

Now, if t = 10, then p = 2 × 100 = 200.
Therefore, equation (1) becomes:

Page 48 of 120

Class XII Chapter 9 – Differential Equations Maths

Hence, the value of r is 6.93%.

Question 21:
In a bank, principal increases continuously at the rate of 5% per year. An amount of Rs

1000 is deposited with this bank, how much will it worth after 10 years .
Answer
Let p and t be the principal and time respectively.
It is given that the principal increases continuously at the rate of 5% per year.

Integrating both sides, we get:

Now, when t = 0, p = 1000.
⇒ 1000 = eC … (2)
At t = 10, equation (1) becomes:

Page 49 of 120

Class XII Chapter 9 – Differential Equations Maths

Hence, after 10 years the amount will worth Rs 1648.

Question 22:
In a culture, the bacteria count is 1,00,000. The number is increased by 10% in 2 hours.
In how many hours will the count reach 2,00,000, if the rate of growth of bacteria is
proportional to the number present?
Answer
Let y be the number of bacteria at any instant t.
It is given that the rate of growth of the bacteria is proportional to the number present.

Integrating both sides, we get:

Let y0 be the number of bacteria at t = 0.
⇒ log y0 = C
Substituting the value of C in equation (1), we get:

Also, it is given that the number of bacteria increases by 10% in 2 hours.
Page 50 of 120

Class XII Chapter 9 – Differential Equations Maths

Substituting this value in equation (2), we get:
Therefore, equation (2) becomes:

Now, let the time when the number of bacteria increases from 100000 to 200000 be t1.
⇒ y = 2y0 at t = t1

From equation (4), we get:

Hence, in hours the number of bacteria increases from 100000 to 200000.

Page 51 of 120

Class XII Chapter 9 – Differential Equations Maths
Question 23:

The general solution of the differential equation
A.
B.
C.
D.
Answer

Integrating both sides, we get:

Hence, the correct answer is A.

Page 52 of 120

Class XII Chapter 9 – Differential Equations Maths
Question 1: Exercise 9.5

Answer
The given differential equation i.e., (x2 + xy) dy = (x2 + y2) dx can be written as:

This shows that equation (1) is a homogeneous equation.
To solve it, we make the substitution as:
y = vx
Differentiating both sides with respect to x, we get:

Substituting the values of v and in equation (1), we get:

Page 53 of 120

Class XII Chapter 9 – Differential Equations Maths

Integrating both sides, we get:

This is the required solution of the given differential equation.
Page 54 of 120

Class XII Chapter 9 – Differential Equations Maths
Question 2:

Answer
The given differential equation is:

Thus, the given equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx
Differentiating both sides with respect to x, we get:

Substituting the values of y and in equation (1), we get:

Integrating both sides, we get:
Page 55 of 120

Class XII Chapter 9 – Differential Equations Maths

This is the required solution of the given differential equation.

Question 3:

Answer
The given differential equation is:

Thus, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx

Substituting the values of y and in equation (1), we get:

Page 56 of 120

Class XII Chapter 9 – Differential Equations Maths

Integrating both sides, we get:

This is the required solution of the given differential equation.
Question 4:
Answer
The given differential equation is:

Therefore, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx

Page 57 of 120

Class XII Chapter 9 – Differential Equations Maths

Substituting the values of y and in equation (1), we get:

Integrating both sides, we get:

This is the required solution of the given differential equation.
Question 5:
Answer
The given differential equation is:

Page 58 of 120

Class XII Chapter 9 – Differential Equations Maths

Therefore, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx

Substituting the values of y and in equation (1), we get:

Integrating both sides, we get:
Page 59 of 120

Class XII Chapter 9 – Differential Equations Maths

This is the required solution for the given differential equation.
Question 6:
Answer

Therefore, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx

Page 60 of 120

Class XII Chapter 9 – Differential Equations Maths

Substituting the values of v and in equation (1), we get:

Integrating both sides, we get:

This is the required solution of the given differential equation.
Question 7:

Answer
The given differential equation is:

Page 61 of 120

Class XII Chapter 9 – Differential Equations Maths

Therefore, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx
Substituting the values of y and in equation (1), we get:

Page 62 of 120

Class XII Chapter 9 – Differential Equations Maths

Integrating both sides, we get:

This is the required solution of the given differential equation.
Page 63 of 120

Class XII Chapter 9 – Differential Equations Maths
Question 8:

Answer

Therefore, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx

Substituting the values of y and in equation (1), we get:

Page 64 of 120

Class XII Chapter 9 – Differential Equations Maths

Integrating both sides, we get:

This is the required solution of the given differential equation.
Question 9:

Answer

Therefore, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx

Page 65 of 120

Class XII Chapter 9 – Differential Equations Maths

Substituting the values of y and in equation (1), we get:

Integrating both sides, we get:
Page 66 of 120

Class XII Chapter 9 – Differential Equations Maths

Therefore, equation (1) becomes:

This is the required solution of the given differential equation.
Question 10:
Answer

Page 67 of 120

Class XII Chapter 9 – Differential Equations Maths

Therefore, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
x = vy

Substituting the values of x and in equation (1), we get:

Integrating both sides, we get:
Page 68 of 120

Class XII Chapter 9 – Differential Equations Maths

This is the required solution of the given differential equation.
Question 11:
Answer

Therefore, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx

Substituting the values of y and in equation (1), we get:

Page 69 of 120

Class XII Chapter 9 – Differential Equations Maths

Integrating both sides, we get:

Now, y = 1 at x = 1.
Substituting the value of 2k in equation (2), we get:
This is the required solution of the given differential equation.

Page 70 of 120

Class XII Chapter 9 – Differential Equations Maths
Question 12:

Answer

Therefore, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx

Substituting the values of y and in equation (1), we get:

Page 71 of 120

Class XII Chapter 9 – Differential Equations Maths

Integrating both sides, we get:

Now, y = 1 at x = 1.

Substituting in equation (2), we get:

This is the required solution of the given differential equation.
Question 13:

Page 72 of 120

Class XII Chapter 9 – Differential Equations Maths
Answer

Therefore, the given differential equation is a homogeneous equation.
To solve this differential equation, we make the substitution as:
y = vx

Substituting the values of y and in equation (1), we get:

Integrating both sides, we get:
Page 73 of 120

Class XII Chapter 9 – Differential Equations Maths

Now,

.

Substituting C = e in equation (2), we get:
This is the required solution of the given differential equation.
Question 14:
Answer

Therefore, the given differential equation is a homogeneous equation.
Page 74 of 120

Class XII Chapter 9 – Differential Equations Maths

To solve it, we make the substitution as:
y = vx

Substituting the values of y and in equation (1), we get:

Integrating both sides, we get:

This is the required solution of the given differential equation.
Now, y = 0 at x = 1.

Substituting C = e in equation (2), we get:
This is the required solution of the given differential equation.

Page 75 of 120

Class XII Chapter 9 – Differential Equations Maths
Question 15:

Answer

Therefore, the given differential equation is a homogeneous equation.
To solve it, we make the substitution as:
y = vx

Substituting the value of y and in equation (1), we get:

Integrating both sides, we get:
Page 76 of 120

Class XII Chapter 9 – Differential Equations Maths

Now, y = 2 at x = 1.
Substituting C = –1 in equation (2), we get:

This is the required solution of the given differential equation.
Question 16:

A homogeneous differential equation of the form can be solved by making the
substitution
A. y = vx
B. v = yx
C. x = vy
D. x = v
Answer

Page 77 of 120

Class XII Chapter 9 – Differential Equations Maths

For solving the homogeneous equation of the form , we need to make the
substitution as x = vy.
Hence, the correct answer is C.

Question 17:
Which of the following is a homogeneous differential equation?
A.

B.

C.

D.
Answer
Function F(x, y) is said to be the homogenous function of degree n, if
F(λx, λy) = λn F(x, y) for any non-zero constant (λ).
Consider the equation given in alternativeD:

Hence, the differential equation given in alternative D is a homogenous equation.
Page 78 of 120

Class XII Chapter 9 – Differential Equations Maths
Question 1: Exercise 9.6

Answer
The given differential equation is
This is in the form of

The solution of the given differential equation is given by the relation,

Page 79 of 120

Class XII Chapter 9 – Differential Equations Maths

Therefore, equation (1) becomes:

This is the required general solution of the given differential equation.
Question 2:
Answer
The given differential equation is
The solution of the given differential equation is given by the relation,

This is the required general solution of the given differential equation.
Question 3:
Answer
The given differential equation is:

Page 80 of 120

Class XII Chapter 9 – Differential Equations Maths

The solution of the given differential equation is given by the relation,

This is the required general solution of the given differential equation.
Question 4:

Answer
The given differential equation is:

The general solution of the given differential equation is given by the relation,

Question 5:
Answer

Page 81 of 120

Class XII Chapter 9 – Differential Equations Maths

By second fundamental theorem of calculus, we obtain

Question 6:
Answer
The given differential equation is:

This equation is in the form of a linear differential equation as:
The general solution of the given differential equation is given by the relation,

Page 82 of 120

Class XII Chapter 9 – Differential Equations Maths

Question 7:
Answer
The given differential equation is:

This equation is the form of a linear differential equation as:

The general solution of the given differential equation is given by the relation,
Page 83 of 120

Class XII Chapter 9 – Differential Equations Maths

Substituting the value of in equation (1), we get:

This is the required general solution of the given differential equation.
Question 8:
Answer

This equation is a linear differential equation of the form:

Page 84 of 120

Class XII Chapter 9 – Differential Equations Maths

The general solution of the given differential equation is given by the relation,

Question 9:
Answer

This equation is a linear differential equation of the form:
The general solution of the given differential equation is given by the relation,

Page 85 of 120

Class XII Chapter 9 – Differential Equations Maths

Question 10:
Answer

This is a linear differential equation of the form:
The general solution of the given differential equation is given by the relation,

Page 86 of 120

Class XII Chapter 9 – Differential Equations Maths

Question 11:
Answer

This is a linear differential equation of the form:
The general solution of the given differential equation is given by the relation,

Page 87 of 120

Class XII Chapter 9 – Differential Equations Maths

Question 12:
Answer

This is a linear differential equation of the form:
The general solution of the given differential equation is given by the relation,

Page 88 of 120

Class XII Chapter 9 – Differential Equations Maths

Question 13:
Answer
The given differential equation is
This is a linear equation of the form:

The general solution of the given differential equation is given by the relation,

Now,
Therefore,

Substituting C = –2 in equation (1), we get:
Page 89 of 120

Class XII Chapter 9 – Differential Equations Maths

Hence, the required solution of the given differential equation is
Question 14:
Answer

This is a linear differential equation of the form:
The general solution of the given differential equation is given by the relation,

Now, y = 0 at x = 1.
Therefore,

Page 90 of 120

Class XII Chapter 9 – Differential Equations Maths
Substituting in equation (1), we get:

This is the required general solution of the given differential equation.
Question 15:

Answer
The given differential equation is
This is a linear differential equation of the form:

The general solution of the given differential equation is given by the relation,

Now,
Therefore, we get:

Page 91 of 120

Class XII Chapter 9 – Differential Equations Maths

Substituting C = 4 in equation (1), we get:

This is the required particular solution of the given differential equation.

Question 16:
Find the equation of a curve passing through the origin given that the slope of the
tangent to the curve at any point (x, y) is equal to the sum of the coordinates of the
point.
Answer
Let F (x, y) be the curve passing through the origin.

At point (x, y), the slope of the curve will be
According to the given information:

This is a linear differential equation of the form:
The general solution of the given differential equation is given by the relation,

Page 92 of 120


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