NCERT Solutions for Maths Class 12 Exercise 6.5

# NCERT Solutions for Maths Class 12 Exercise 6.5

## NCERT Solutions for Maths Class 12 Exercise 6.5

Hello Students! Welcome to maths-formula.com. In this post, you will find the complete NCERT Solutions for Maths Class 12 Exercise 6.5.

You can download the PDF of NCERT Books Maths Chapter 6 for your easy reference while studying NCERT Solutions for Maths Class 12 Exercise 6.5.

Class 12th is a very crucial stage of your student’s life, since you take all important decisions about your career on this stage. Mathematics plays a vital role to take decision for your career because if you are good in mathematics, you can choose engineering and technology field as your career.

NCERT Solutions for Maths Class 12 Exercise 6.5 helps you to solve each and every problem with step by step explanation which makes you strong in mathematics.

All the schools affiliated with CBSE, follow the NCERT books for all subjects. You can check your syllabus from NCERT Syllabus for Mathematics Class 12.

NCERT Solutions for Maths Class 12 Exercise 6.5 are prepared by the experienced teachers of CBSE board. If you are preparing for JEE Mains and NEET level exams, then it will definitely make your foundation strong.

If you want to recall All Maths Formulas for Class 12, you can find it by clicking this link.

If you want to recall All Maths Formulas for Class 11, you can find it by clicking this link.

NCERT Solutions for Maths Class 12 Exercise 6.1

NCERT Solutions for Maths Class 12 Exercise 6.2

NCERT Solutions for Maths Class 12 Exercise 6.3

NCERT Solutions for Maths Class 12 Exercise 6.4

## NCERT Solutions for Maths Class 12 Exercise 6.5

Maths Class 12 Ex 6.5 Question 1.

Find the maximum and minimum values, if any, of the following functions given by
(i) f(x) = (2x – 1)² + 3
(ii) f(x) = 9x² + 12x + 2
(iii) f(x) = – (x – 1)² + 10
(iv) g(x) = x3 + 1

Solution:
(i) We have, f(x) = (2x – 1)² + 3

Minimum value of (2x – 1)² is zero.
Minimum value of (2x – 1)² + 3 is 3.
Clearly, it does not have maximum value.

(ii) We have, f(x) = 9x² + 12x + 2
f(x) = (3x + 2)² – 2
Minimum value of (3 + 2)² is zero.
Minimum value of (3x + 2)² – 2 is –2.
Clearly, f(x) does not have finite maximum value.

(iii) We have, f(x) = – (x – 1)² + 10
Maximum value of –(x – 1)² is zero.
Maximum value of f(x) = – (x – 1)² + 10 is 10.
Clearly, f(x) does not have finite minimum value.

(iv) We have, g(x) = x3 + 1

As x → ∞, g(x) → ∞; Also, x → -∞, g(x) → -∞
Thus, there is no maximum or minimum value of f(x).

Maths Class 12 Ex 6.5 Question 2.

Find the maximum and minimum values, if any, of the following functions given by
(i) f(x) = |x + 2| – 1
(ii) g(x) = –|x + 1| + 3
(iii) h(x) = sin 2x + 5
(iv) f(x) = |sin(4x + 3)|
(v) h(x) = x + 1, x
(–1, 1)

Solution:
(i) We have, f(x) = |x + 2| – 1 x R
Now, |x + 2| ≥ 0
x R
|x + 2| – 1 ≥ – 1
x R,
So, –1 is the minimum value of f(x).
Now, f(x) = –1
|x + 2| – 1 = –1
|x + 2| = 0
x = – 2

(ii) We have, g(x) = = –|x + 1| + 3 x R
Now, |x + 1| ≥ 0
x R     –|x + 1| ≤ 0 x R
–|x + 1| + 3 ≤ 3
x R
So, 3 is the maximum value of g(x).
Now, g(x) = 3
–|x + 1| + 3 = 3
|x + 1| = 0
x = –1

(iii) We have, h(x) = sin 2x + 5

Since –1 ≤ sin 2x ≤ 1 x R

Adding 5 to all terms, –1 + 5 ≤ sin 2x + 5 ≤ 1 + 5 x R

4 ≤ sin 2x + 5 ≤ 6 x R

Thus, the maximum value of f(x) is 6 and the minimum value is 4.

(iv) We have, f(x) = |sin 4x + 3|
Since –1 ≤ sin 4x ≤ 1
x R

Adding 3 to all terms, –1 + 3 ≤ sin 2x + 3 ≤ 1 + 3 x R

2 ≤ sin 2x + 3 ≤ 4 x R

Thus, the maximum value of f(x) is 4 and the minimum value is 2.

(v) We have, h(x) = x + 1, x (–1, 1)

Since –1 < x < 1

Adding 1 to all terms, –1 + 1 ≤ x + 1 ≤ 1 + 1

0 ≤ x + 1 ≤ 2

Thus, the maximum value of h(x) is 2 and the minimum value is 0. But these values do not exist in the interval (–1, 1).

Therefore, the function h(x) has neither minimum value nor maximum value in (–1, 1).

Maths Class 12 Ex 6.5 Question 3.

Find the local maxima and local minima, if any, of the following functions. Find also the local maximum and the local minimum values, as the case may be:
(i) f(x) = x2
(ii) g(x) = x3 – 3x
(iii) h(x) = sin x + cos x, 0 < x < Ï€/2
(iv) f(x) = sin x – cos x, 0 < x < 2Ï€
(v) f(x) = x– 6x2 + 9x + 15
(vi) g(x) = x/2 + 2/x, x > 0
(vii) g(x) = 1/(x2 + 2)
(viii) f(x) =
, x > 0

Solution:

(i) We have, f(x) = x² f’(x) = 2x and f’’(x) = 2
Now, f'(x) = 0
2x = 0 i.e., x = 0
When x = 0, f’’(x) = 2 (+ve)

f(x) has a local minimum at x = 0 and local minimum value f(0) = 0.

Maths Class 12 Ex 6.5 Question 4.

Prove that the following functions do not have maxima or minima:
(i) f(x) = ex
(ii) f(x) = log x
(iii) h(x) = x3 + x2 + x + 1

Solution:
(i) We have, f(x) = ex f'(x) = ex;
Since f’(x) ≠ 0 for any value of x.
So, f(x) = ex does not have a maxima or minima.

(ii) We have, f(x) = log x f’(x) = 1/x;

Clearly f’(x) ≠ 0 for any value of x.
So, f(x) = log x does not have a maxima or minima.

(iii) We have, f(x) = x3 + x2 + x + 1
f’(x) = 3x2 + 2x + 1
Now, f’(x) = 0
3x2 + 2x + 1 = 0

i.e. f'(x) = 0 at imaginary points
i.e. f'(x) ≠ 0 for any real value of x.
Hence, there is neither maxima nor minima.

Maths Class 12 Ex 6.5 Question 5.

Find the absolute maximum value and the absolute minimum value of the following functions in the given intervals:
(i) f(x) = x3, x
[–2, 2]
(ii) f(x) = sin x + cos x, x
[0, Ï€]
(iii) f(x) = 4x – x2/2, x
[–2, 9/2]
(iv) f(x) = (x – 1)2 + 3, x
[–3, 1]

Solution:
(i) We have, f(x) = x3 in [ –2, 2]
f'(x) = 3x²; Now, f’(x) = 0 at x = 0, f(0) = 0
Now, f(–2) = (–2)3 = – 8; f(0) = (0)² = 0 and f(2) = (2)3 = 8
Hence, the absolute maximum value of f(x) is 8 which is attained at x = 2 and absolute minimum value of f(x) is –8 which is attained at x = –2.

(ii) We have, f(x) = sin x + cos x in [0, Ï€]
f’(x) = cos x – sin x for extreme values f’(x) = 0

Maths Class 12 Ex 6.5 Question 6.

Find the maximum profit that a company can make, if the profit function is given by p(x) = 41 – 24x – 18x²

Solution:
Profit function is given by p(x) = 41 – 24x – 18x²
p'(x) = – 24 – 36x = –12(2 + 3x)
For maxima and minima, p'(x) = 0
Now, p'(x) = 0
–12(2 + 3x) = 0
x = –2/3
p'(x) changes sign from +ve to -ve.
p(x) has maximum value at x = –2/3
Maximum Profit = 41 + 16 – 8 = 49.

Maths Class 12 Ex 6.5 Question 7.

Find both the maximum value and the minimum value of 3x4 – 8x3 + 12x2 – 48x + 25 on the interval [0, 3].

Solution:
Let f(x) = 3x4 – 8x3 + 12x2 – 48x + 25
f'(x) = 12x3 – 24x2 + 24x – 48
= 12(x2 + 2)(x – 2)
For maxima and minima, f'(x) = 0
12(x2 + 2)(x – 2) = 0
x = 2
Now, we find f(x) at x = 0, 2 and 3, f(0) = 25,
f(2) = 3(24) – 8(23) + 12(22) – 48(2) + 25 = –39
and f(3) = (34) – 8(33) + 12(32) – 48(3) + 25
= 243 – 216 + 108 – 144 + 25 = 16
Hence, at x = 0, maximum value = 25
and at x = 2, minimum value = –39.

Maths Class 12 Ex 6.5 Question 8.

At what points in the interval [0, 2Ï€] does the function sin 2x attain its maximum value?

Solution:
We have, f(x) = sin 2x in [0, 2Ï€] f’(x) = 2 cos 2x
For maxima and minima, f’(x) = 0
cos 2x = 0

Maths Class 12 Ex 6.5 Question 9.

What is the maximum value of the function sin x + cos x?

Solution:
Consider the interval [0, 2Ï€].
Let f(x) = sin x + cos x,
f’(x) = cos x – sin x
For maxima and minima, f’(x) = 0

Maths Class 12 Ex 6.5 Question 10.

Find the maximum value of 2x3 – 24x + 107 in the interval [1, 3]. Find the maximum value of the same function in [–3, –1].

Solution:
f(x) = 2x3 – 24x + 107
f’(x) = 6x2 – 24,
For maxima and minima, f'(x) = 0
x = ±2
For the interval [ 1, 3], we find the values of f(x) at x = 1, 2, 3;

f(1) = 85, f(2) = 75, f(3) = 89
Hence, maximum value of f(x) = 89 at x = 3.
For the interval [–3, –1], we find the values of f(x) at x = – 3, – 2, – 1;
f(–3) = 125;
f(–2) = 139
f(–1) = 129
Maximum value of f(x) = 139 at x = –2.

Maths Class 12 Ex 6.5 Question 11.

It is given that at x = 1, the function x4 – 62x2 + ax + 9 attains its maximum value, on the interval [0, 2]. Find the value of a.

Solution:
We have, f(x) = x4 – 62x2 + ax + 9
f’(x) = 4x3 – 124x + a
Now, f’(x) = 0 at x = 1
4 – 124 + a = 0
a = 120
Now, f”(x) = 12x2 – 124
At x = 1, f”(1) = 12 – 124 = – 112 < 0
f(x) has a maximum value at x = 1 when a = 120.

Maths Class 12 Ex 6.5 Question 12.

Find the maximum and minimum values of x + sin 2x on [0, 2Ï€].

Solution: We have, f(x) = x + sin 2x on [0, 2Ï€]
f’(x) = 1 + 2 cos 2x
For maxima and minima f’(x) = 0

Maths Class 12 Ex 6.5 Question 13.

Find two numbers whose sum is 24 and whose product is as large as possible.

Solution:
Let the required numbers are x and (24 – x).
Their product, p = x(24 – x) = 24x – x²
Now, dp/dx = 0
24 – 2x = 0 x = 12
Also, d2p/dx2 = –2 < 0
p is maximum at x = 12.
Hence, the required numbers are 12 and (24 – 12) i.e., 12.

Maths Class 12 Ex 6.5 Question 14.

Find two positive numbers x and y such that x + y = 60 and xy3 is maximum.

Solution:
We have, x + y = 60
y = 60 – x    … (i)
Let z = xy3

z = x(60 – x)3

dz/dx = x[3(60 – x)2(–1)] + (60 – x)3

= (60 – x)2[–3x + 60 – x]

= (60 – x)2(60 – 4x)

Maths Class 12 Ex 6.5 Question 15.

Find two positive numbers x and y such that their sum is 35 and the product x2y5 is a maximum.

Solution:
We have, x + y = 35 y = 35 – x
Product, P = x2y5
= x2(35 – x)5

Maths Class 12 Ex 6.5 Question 16.

Find two positive numbers whose sum is 16 and the sum of whose cubes is minimum.

Solution:
Let two numbers be x and 16 – x.

Hence, the required numbers are 8 and (16 – 8) i.e., 8 and 8.

Maths Class 12 Ex 6.5 Question 17.

A square piece of tin of side 18 cm is to be made into a box without top, by cutting a square from each corner and folding up the flaps to form the box. What should be the side of the square to be cut off so that the volume of the box is the maximum possible?

Solution:
Let each side of the square to be cut off be x cm.
For the box, length = 18 – 2x; breadth = 18 – 2x and height = x

Maths Class 12 Ex 6.5 Question 18.

A rectangular sheet of tin 45 cm by 24 cm is to be made into a box without top, by cutting off square from each corner and folding up the flaps. What should be the side of the square to be cut off so that the volume of the box is maximum?

Solution:
Let each side of the square cut off from each corner be x cm.
Sides of the rectangular box are (45 – 2x), (24 – 2x) and x cm.

Maths Class 12 Ex 6.5 Question 19.

Show that of all the rectangles inscribed in a given fixed circle, the square has the maximum area.

Solution:
Let the length and the breadth of the rectangle inscribed in a circle of radius a be x and y respectively.
x² + y² = (2a)² x² + y² = 4a²     … (i)
Perimeter = 2(x + y)

Maths Class 12 Ex 6.5 Question 20.

Show that the right circular cylinder of given surface and maximum volume is such that its height is equal to the diameter of the base.

Solution:
Let S be the given surface area of the closed cylinder whose radius is r and height is h. Let V be its Volume. Then

Maths Class 12 Ex 6.5 Question 21.

Of all the closed cylindrical cans (right circular), of a given volume of 100 cubic centimetres, find the dimensions of the can which has the minimum surface area?

Solution:
Let r be the radius and h be the height of cylindrical can.

Maths Class 12 Ex 6.5 Question 22.

A wire of length 28 m is to be cut into two pieces. One of the pieces is to be made into a square and the other into a circle. What should be the length of the two pieces so that the combined area of the square and the circle is minimum?

Solution:
Let the length of one part be x m, then the other part = (28 – x) m.
Let the part of the length x m be converted into a circle of radius r.

Maths Class 12 Ex 6.5 Question 23.

Prove that the volume of the largest cone that can be inscribed in a sphere of radius R is 8/27 of the volume of the sphere.

Solution:
Let a cone VAB of greatest volume be inscribed in the sphere. Let AOC = Î¸.
AC, the radius of the base of the cone = R sin Î¸ and VC = VO + OC = R(1 + cos Î¸)
= R + R cos Î¸ = height of the cone

Maths Class 12 Ex 6.5 Question 24.

Show that the right circular cone of least curved surface and given volume has an altitude equal to √2 time the radius of the base.

Solution:
Let r and h be the radius and the height of the cone.

Maths Class 12 Ex 6.5 Question 25.

Show that the semi-vertical angle of the cone of the maximum volume and of given slant height is tan-1 √2.

Solution:
Let v be the volume, l be the slant height and a be the semi-vertical angle of a cone.

Maths Class 12 Ex 6.5 Question 26.

Show that semi-vertical angle of right circular cone of given surface area and maximum volume is sin-1 (1/3).

Solution:
Let r be the radius, l be the slant height and h be the height of the cone of given surface area s. Then,

Choose the correct answer in the Exercises 27 and 29.

Maths Class 12 Ex 6.5 Question 27.

The point on the curve x² = 2y which is nearest to the point (0, 5) is:
(A) (2√2, 4)
(B) (2√2, 0)
(C) (0, 0)
(D) (2, 2)

Solution:
(A) Let P(x, y) be a point on the curve. The other point is A(0, 5).
z = PA² = x² + y² + 25 – 10y          [
x² = 2y]

Maths Class 12 Ex 6.5 Question 28.

For all real values of x, the minimum value of (1 – x + x2)/(1 + x + x2is:
(A) 0
(B) 1
(C) 3
(D) 1/3

Solution:

Maths Class 12 Ex 6.5 Question 29.

The maximum value of [x(x – 1) + 1]1/3, 0 ≤ x ≤ 1 is:
(A) (1/3)1/3
(B) 1/2
(C) 1
(D) 0

Solution: