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# RD Sharma solutions for Class 12 Mathematics chapter 17 - Increasing and Decreasing Functions

## Chapter 17: Increasing and Decreasing Functions

#### Chapter 17: Increasing and Decreasing Functions solutions [Page 10]

Q 1 | Page 10

Prove that the function f(x) = loge x is increasing on (0, ∞) ?

Q 2 | Page 10

Prove that the function f(x) = loga x is increasing on (0, ∞) if a > 1 and decreasing on (0, ∞), if 0 < a < 1 ?

Q 3 | Page 10

Prove that f(x) = ax + b, where a, b are constants and a > 0 is an increasing function on R ?

Q 4 | Page 10

Prove that f(x) = ax + b, where a, b are constants and a < 0 is a decreasing function on R ?

Q 5 | Page 10

Show that f(x) = $\frac{1}{x}$ is a decreasing function on (0, ∞) ?

Q 6 | Page 10

Show that f(x) = $\frac{1}{1 + x^2}$ decreases in the interval [0, ∞) and increases in the interval (−∞, 0] ?

Q 7 | Page 10

Show that f(x) = $\frac{1}{1 + x^2}$ is neither increasing nor decreasing on R ?

Q 8 | Page 10

Without using the derivative, show that the function f (x) = | x | is.
(a) strictly increasing in (0, ∞)
(b) strictly decreasing in (−∞, 0) .

Q 9 | Page 10

Without using the derivative show that the function f (x) = 7x − 3 is strictly increasing function on R ?

#### Chapter 17: Increasing and Decreasing Functions solutions [Pages 33 - 35]

Q 1.01 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = 10 − 6x − 2x2  ?

Q 1.02 | Page 33

Find the interval in which the following function are increasing or decreasing  f(x) = x2 + 2x − 5  ?

Q 1.03 | Page 33

Find the interval in which the following function are increasing or decreasing  f(x) = 6 − 9x − x2  ?

Q 1.04 | Page 33

Find the interval in which the following function are increasing or decreasing   f(x) = 2x3 − 12x2 + 18x + 15 ?

Q 1.05 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = 5 + 36x + 3x2 − 2x?

Q 1.06 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = 8 + 36x + 3x2 − 2x?

Q 1.07 | Page 33

Find the interval in which the following function are increasing or decreasing  f(x) = 5x3 − 15x2 − 120x + 3 ?

Q 1.08 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = x3 − 6x2 − 36x + 2 ?

Q 1.09 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = 2x3 − 15x2 + 36x + 1 ?

Q 1.1 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = 2x3 + 9x2 + 12x + 20  ?

Q 1.11 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = 2x3 − 9x2 + 12x − 5 ?

Q 1.12 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = 6 + 12x + 3x2 − 2x3 ?

Q 1.13 | Page 33

Find the interval in which the following function are increasing or decreasing  f(x) = 2x3 − 24x + 107  ?

Q 1.14 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = −2x3 − 9x2 − 12x + 1  ?

Q 1.15 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = (x − 1) (x − 2)?

Q 1.16 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = x3 − 12x2 + 36x + 17 ?

Q 1.17 | Page 33

Find the interval in which the following function are increasing or decreasing  f(x) = 2x3 − 24x + 7 ?

Q 1.18 | Page 33

Find the interval in which the following function are increasing or decreasing $f\left( x \right) = \frac{3}{10} x^4 - \frac{4}{5} x^3 - 3 x^2 + \frac{36}{5}x + 11$ ?

Q 1.19 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = x4 − 4x ?

Q 1.2 | Page 33

Find the interval in which the following function are increasing or decreasing $f\left( x \right) = \frac{x^4}{4} + \frac{2}{3} x^3 - \frac{5}{2} x^2 - 6x + 7$ ?

Q 1.21 | Page 33

Find the interval in which the following function are increasing or decreasing  f(x) = x4 − 4x3 + 4x2 + 15 ?

Q 1.22 | Page 33

Find the interval in which the following function are increasing or decreasing  f(x) =  $5 x^\frac{3}{2} - 3 x^\frac{5}{2}$  x > 0 ?

Q 1.23 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = x8 + 6x2  ?

Q 1.24 | Page 33

Find the interval in which the following function are increasing or decreasing f(x) = x3 − 6x2 + 9x + 15 ?

Q 1.25 | Page 33

Find the interval in which the following function are increasing or decreasing $f\left( x \right) = \left\{ x(x - 2) \right\}^2$ ?

Q 1.26 | Page 33

Find the interval in which the following function are increasing or decreasing $f\left( x \right) = 3 x^4 - 4 x^3 - 12 x^2 + 5$ ?

Q 1.27 | Page 33

Find the interval in which the following function are increasing or decreasing $f\left( x \right) = \frac{3}{2} x^4 - 4 x^3 - 45 x^2 + 51$ ?

Q 1.28 | Page 33

Find the interval in which the following function are increasing or decreasing $f\left( x \right) = \log\left( 2 + x \right) - \frac{2x}{2 + x}, x \in R$ ?

Q 2 | Page 34

Determine the values of x for which the function f(x) = x2 − 6x + 9 is increasing or decreasing. Also, find the coordinates of the point on the curve y = x2 − 6x + 9 where the normal is parallel to the line y = x + 5 ?

Q 3 | Page 34

Find the intervals in which f(x) = sin x − cos x, where 0 < x < 2π is increasing or decreasing ?

Q 4 | Page 34

Show that f(x) = e2x is increasing on R ?

Q 5 | Page 34

Show that f(x) = e1/x, x ≠ 0 is a decreasing function for all x ≠ 0 ?

Q 6 | Page 34

Show that f(x) = loga x, 0 < a < 1 is a decreasing function for all x > 0 ?

Q 7 | Page 34

Show that f(x) = sin x is increasing on (0, π/2) and decreasing on (π/2, π) and neither increasing nor decreasing in (0, π) ?

Q 8 | Page 34

Show that f(x) = log sin x is increasing on (0, π/2) and decreasing on (π/2, π) ?

Q 9 | Page 34

Show that f(x) = x − sin x is increasing for all x ∈ R ?

Q 10 | Page 34

Show that f(x) = x3 − 15x2 + 75x − 50 is an increasing function for all x ∈ R ?

Q 11 | Page 34

Show that f(x) = cos2 x is a decreasing function on (0, π/2) ?

Q 12 | Page 34

Show that f(x) = sin x is an increasing function on (−π/2, π/2) ?

Q 13 | Page 34

Show that f(x) = cos x is a decreasing function on (0, π), increasing in (−π, 0) and neither increasing nor decreasing in (−π, π) ?

Q 14 | Page 34

Show that f(x) = tan x is an increasing function on (−π/2, π/2) ?

Q 15 | Page 34

Show that f(x) = tan−1 (sin x + cos x) is a decreasing function on the interval (π/4, π/2) ?

Q 16 | Page 34

Show that the function f(x) = sin (2x + π/4) is decreasing on (3π/8, 5π/8) ?

Q 17 | Page 34

Show that the function f(x) = cot $-$ l(sinx + cosx) is decreasing on $\left( 0, \frac{\pi}{4} \right)$ and increasing on $\left( 0, \frac{\pi}{4} \right)$ ?

Q 18 | Page 34

Show that f(x) = (x − 1) ex + 1 is an increasing function for all x > 0 ?

Q 19 | Page 34

Show that the function x2 − x + 1 is neither increasing nor decreasing on (0, 1) ?

Q 20 | Page 34

Show that f(x) = x9 + 4x7 + 11 is an increasing function for all x ∈ R ?

Q 21 | Page 35

Prove that the function f(x) = x3 − 6x2 + 12x − 18 is increasing on R ?

Q 22 | Page 35

State when a function f(x) is said to be increasing on an interval [a, b]. Test whether the function f(x) = x2 − 6x + 3 is increasing on the interval [4, 6] ?

Q 23 | Page 35

Show that f(x) = sin x − cos x is an increasing function on (−π/4, π/4) ?

Q 24 | Page 35

Show that f(x) = tan−1 x − x is a decreasing function on R ?

Q 25 | Page 35

Determine whether f(x) = −x/2 + sin x is increasing or decreasing on (−π/3, π/3) ?

Q 26 | Page 35

Find the intervals in which f(x) = log (1 + x) −$\frac{x}{1 + x}$ is increasing or decreasing ?

Q 27 | Page 35

Find the intervals in which f(x) = (x + 2) e−x is increasing or decreasing ?

Q 28 | Page 35

Show that the function f given by f(x) = 10x is increasing for all x ?

Q 29 | Page 35

Prove that the function f given by f(x) = x − [x] is increasing in (0, 1) ?

Q 30.1 | Page 35

Prove that the following function is increasing on R f $(x) =$3 $x^5$ + 40 $x^3$ + 240$x$ ?

Q 30.2 | Page 35

Prove that the following function is increasing on R f $f\left( x \right) = 4 x^3 - 18 x^2 + 27x - 27$ ?

Q 31 | Page 35

Prove that the function f given by f(x) = log cos x is strictly increasing on (−π/2, 0) and strictly decreasing on (0, π/2) ?

Q 32 | Page 35

Prove that the function f given by f(x) = x3 − 3x2 + 4x is strictly increasing on R ?

Q 33 | Page 35

Prove that the function f(x) = cos x is:
(i) strictly decreasing in (0, π)
(ii) strictly increasing in (π, 2π)
(iii) neither increasing nor decreasing in (0, 2π).

Q 34 | Page 35

Show that f(x) = x2 − x sin x is an increasing function on (0, π/2) ?

Q 35 | Page 35

Find the value(s) of a for which f(x) = x3 − ax is an increasing function on R ?

Q 36 | Page 35

Find the values of b for which the function f(x) = sin x − bx + c is a decreasing function on R ?

Q 37 | Page 35

Show that f(x) = x + cos x − a is an increasing function on R for all values of a ?

Q 38 | Page 35

Let f defined on [0, 1] be twice differentiable such that | f"(x) | ≤ 1 for all x ∈ [0, 1]. If f(0) = f(1), then show that | f'(x) | < 1 for all x ∈ [ 0, 1] ?

Q 39.1 | Page 35

Find the interval in which f(x) is increasing or decreasing f(x) = x|x|, x $\in$ R ?

Q 39.2 | Page 35

Find the interval in which f(x) is increasing or decreasing f(x) = sinx + |sin x|, 0 < x $\leq 2\pi$ ?

Q 39.3 | Page 35

Find the interval in which f(x) is increasing or decreasing f(x) = sinx(1 + cosx), 0 < x < $\frac{\pi}{2}$ ?

#### Chapter 17: Increasing and Decreasing Functions solutions [Pages 39 - 40]

Q 1 | Page 39

What are the values of 'a' for which f(x) = ax is increasing on R ?

Q 2 | Page 39

What are the values of 'a' for which f(x) = ax is decreasing on R ?

Q 3 | Page 39

Write the set of values of 'a' for which f(x) = loga x is increasing in its domain ?

Q 4 | Page 39

Write the set of values of 'a' for which f(x) = loga x is decreasing in its domain ?

Q 5 | Page 39

Find 'a' for which f(x) = a (x + sin x) + a is increasing on R ?

Q 6 | Page 39

Find the values of 'a' for which the function f(x) = sin x − ax + 4 is increasing function on R ?

Q 7 | Page 39

Find the set of values of 'b' for which f(x) = b (x + cos x) + 4 is decreasing on R ?

Q 8 | Page 40

Find the set of values of 'a' for which f(x) = x + cos x + ax + b is increasing on R ?

Q 9 | Page 40

Write the set of values of k for which f(x) = kx − sin x is increasing on R ?

Q 10 | Page 40

If g (x) is a decreasing function on R and f(x) = tan−1 [g (x)]. State whether f(x) is increasing or decreasing on R ?

Q 11 | Page 40

Write the set of values of a for which the function f(x) = ax + b is decreasing for all x ∈ R ?

Q 12 | Page 40

Write the interval in which f(x) = sin x + cos x, x ∈ [0, π/2] is increasing ?

Q 13 | Page 40

State whether f(x) = tan x − x is increasing or decreasing its domain ?

Q 14 | Page 40

Write the set of values of a for which f(x) = cos x + a2 x + b is strictly increasing on R ?

#### Chapter 17: Increasing and Decreasing Functions solutions [Pages 40 - 42]

Q 1 | Page 40

The interval of increase of the function f(x) = x − ex + tan (2π/7) is
(a) (0, ∞)
(b) (−∞, 0)
(c) (1, ∞)
(d) (−∞, 1)

Q 2 | Page 40

The function f(x) = cot−1 x + x increases in the interval
(a) (1, ∞)
(b) (−1, ∞)
(c) (−∞, ∞)
(d) (0, ∞)

Q 3 | Page 40

The function f(x) = xx decreases on the interval
(a) (0, e)
(b) (0, 1)
(c) (0, 1/e)
(d) none of these

Q 4 | Page 40

The function f(x) = 2 log (x − 2) − x2 + 4x + 1 increases on the interval
(a) (1, 2)
(b) (2, 3)
(c) (1, 3)
(d) (2, 4)

Q 5 | Page 40

If the function f(x) = 2x2 − kx + 5 is increasing on [1, 2], then k lies in the interval
(a) (−∞, 4)
(b) (4, ∞)
(c) (−∞, 8)
(d) (8, ∞)

Q 6 | Page 40

Let f(x) = x3 + ax2 + bx + 5 sin2x be an increasing function on the set R. Then, a and bsatisfy
(a) a2 − 3b − 15 > 0
(b) a2 − 3b + 15 > 0
(c) a2 − 3b + 15 < 0
(d) a > 0 and b > 0

Q 7 | Page 40

The function $f\left( x \right) = \log_e \left( x^3 + \sqrt{x^6 + 1} \right)$ is of the following types:
(a) even and increasing
(b) odd and increasing
(c) even and decreasing
(d) odd and decreasing

Q 8 | Page 40

If the function f(x) = 2 tan x + (2a + 1) loge | sec x | + (a − 2) x is increasing on R, then
(a) a ∈ (1/2, ∞)
(b) a ∈ (−1/2, 1/2)
(c) a = 1/2
(d) a ∈ R

Q 9 | Page 40

Let $f\left( x \right) = \tan^{- 1} \left( g\left( x \right) \right),$,where g (x) is monotonically increasing for 0 < x < $\frac{\pi}{2} .$ Then, f(x) is
(a) increasing on (0, π/2)
(b) decreasing on (0, π/2)
(c) increasing on (0, π/4) and decreasing on (π/4, π/2)
(d) none of these

Q 10 | Page 40

Let f(x) = x3 − 6x2 + 15x + 3. Then,
(a) f(x) > 0 for all x ∈ R
(b) f(x) > f(x + 1) for all x ∈ R
(c) f(x) is invertible
(d) none of these

Q 11 | Page 41

The function f(x) = x2 e−x is monotonic increasing when
(a) x ∈ R − [0, 2]
(b) 0 < x < 2
(c) 2 < x < ∞
(d) x < 0

Q 12 | Page 41

Function f(x) = cos x − 2 λ x is monotonic decreasing when
(a) λ > 1/2
(b) λ < 1/2
(c) λ < 2
(d) λ > 2

Q 13 | Page 41

In the interval (1, 2), function f(x) = 2 | x − 1 | + 3 | x − 2 | is
(a) monotonically increasing
(b) monotonically decreasing
(c) not monotonic
(d) constant

Q 14 | Page 41

Function f(x) = x3 − 27x + 5 is monotonically increasing when
(a) x < −3
(b) | x | > 3
(c) x ≤ −3
(d) | x | ≥ 3

Q 15 | Page 41

Function f(x) = 2x3 − 9x2 + 12x + 29 is monotonically decreasing when
(a) x < 2
(b) x > 2
(c) x > 3
(d) 1 < x < 2

Q 16 | Page 41

If the function f(x) = kx3 − 9x2 + 9x + 3 is monotonically increasing in every interval, then
(a) k < 3
(b) k ≤ 3
(c) k > 3
(d) k ≥ 3

Q 17 | Page 41

f(x) = 2x − tan−1 x − log $\left\{ x + \sqrt{x^2 + 1} \right\}$ is monotonically increasing when

(a) x > 0
(b) x < 0
(c) x ∈ R
(d) x ∈ R − {0}

Q 18 | Page 41

Function f(x) = | x | − | x − 1 | is monotonically increasing when
(a) x < 0
(b) x > 1
(c) x < 1
(d) 0 < x < 1

Q 19 | Page 41

Every invertible function is
(a) monotonic function
(b) constant function
(c) identity function
(d) not necessarily monotonic function

Q 20 | Page 41

In the interval (1, 2), function f(x) = 2 | x − 1 | + 3 | x − 2 | is
(a) increasing
(b) decreasing
(c) constant
(d) none of these

Q 21 | Page 41

If the function f(x) = cos |x| − 2ax + b increases along the entire number scale, then

(a) a = b
(b) $a = \frac{1}{2}b$

(c) $a \leq - \frac{1}{2}$

(d)  $a > - \frac{3}{2}$

Q 22 | Page 41

The function $f\left( x \right) = \frac{x}{1 + \left| x \right|}$ is

(a) strictly increasing
(b) strictly decreasing
(c) neither increasing nor decreasing
(d) none of these

Q 23 | Page 41

The function $f\left( x \right) = \frac{\lambda \sin x + 2 \cos x}{\sin x + \cos x}$ is increasing, if

(a) λ < 1
(b) λ > 1
(c) λ < 2
(d) λ > 2

Q 24 | Page 41

Function f(x) = ax is increasing on R, if
(a) a > 0
(b) a < 0
(c) 0 < a < 1
(d) a > 1

Q 25 | Page 41

Function f(x) = loga x is increasing on R, if
(a) 0 < a < 1
(b) a > 1
(c) a < 1
(d) a > 0

Q 26 | Page 41

Let ϕ(x) = f(x) + f(2a − x) and f"(x) > 0 for all x ∈ [0, a]. Then, ϕ (x)
(a) increases on [0, a]
(b) decreases on [0, a]
(c) increases on [−a, 0]
(d) decreases on [a, 2a]

Q 27 | Page 41

If the function f(x) = x2 − kx + 5 is increasing on [2, 4], then
(a) k ∈ (2, ∞)
(b) k ∈ (−∞, 2)
(c) k ∈ (4, ∞)
(d) k ∈ (−∞, 4).

Q 28 | Page 41

The function f(x) = −x/2 + sin x defined on [−π/3, π/3] is
(a) increasing
(b) decreasing
(c) constant
(d) none of these

Q 29 | Page 42

If the function f(x) = x3 − 9kx2 + 27x + 30 is increasing on R, then
(a) −1 ≤ k < 1
(b) k < −1 or k > 1
(c) 0 < k < 1
(d) −1 < k < 0

Q 30 | Page 42

The function f(x) = x9 + 3x7 + 64 is increasing on
(a) R
(b) (−∞, 0)
(c) (0, ∞)
(d) R0

#### Chapter 17: Increasing and Decreasing Functions solutions [Page 7]

Q 1 | Page 7

f ( $x$)=4$x^2$-4  $x$ + 4 on R .

Q 2 | Page 7

f(x)=(x-1)2+2 on R ?

Q 3 | Page 7

f(x)=| x+2 | on R .

Q 4 | Page 7

f(x)=sin 2x+5 on R .

Q 5 | Page 7

f(x) = | sin 4x+3 | on R ?

Q 6 | Page 7

f(x)=2x3 +5 on R .

Q 7 | Page 7

f (x) = $-$ | x + 1 | + 3 on R .

Q 8 | Page 7

f(x) = 16x2 $-$ 16x + 28 on R ?

Q 9 | Page 7

f(x) = x$-$ 1 on R .

#### Chapter 17: Increasing and Decreasing Functions solutions [Page 16]

Q 1 | Page 16

f(x) = (x $-$ 5)4.

Q 2 | Page 16

f(x) = x$-$ 3x .

Q 3 | Page 16

f(x) = x3  (x $-$ 1).

Q 4 | Page 16

f(x) =  (x $-$ 1) (x+2)2

Q 5 | Page 16

f(x) = $\frac{1}{x^2 + 2}$ .

Q 6 | Page 16

f(x) =  x$-$ 6x2 + 9x + 15 .

Q 7 | Page 16

f(x) = sin 2x, 0<x< $\pi$ .

Q 8 | Page 16

f(x) =  sin x $-$ cos x, 0 < x<2 $\pi$ .

Q 9 | Page 16

f(x) =  cos x, 0<x< $\pi$ .

Q 10 | Page 16

f(x) = sin 2x $-$ x, $- \frac{\pi}{2} < \frac{<}{}x\frac{<}{}\frac{\pi}{2}$ .

Q 11 | Page 16

f(x) = 2sin x$-$ x, $- \frac{\pi}{2} < \frac{<}{}x\frac{<}{}\frac{\pi}{2}$ .

Q 12 | Page 16

f(x) =$x\sqrt{1 - x} , x > 0$.

Q 13 | Page 16

f(x) = x3 (2x $-$ 1)3.

Q 14 | Page 16

f(x) =$\frac{x}{2} + \frac{2}{x} , x > 0$ .

#### Chapter 17: Increasing and Decreasing Functions solutions [Page 31]

Q 1.01 | Page 31

f(x) = x4 $-$ 62x2 + 120x + 9.

Q 1.02 | Page 31

f(x) = x3$-$ 6x2 + 9x + 15

Q 1.03 | Page 31

f(x) = (x $-$ 1) (x+2)2.

Q 1.04 | Page 31

f(x) = 2/x $-$ 2/x2 , x>0 .

Q 1.05 | Page 31

f(x) = xex.

Q 1.06 | Page 31

f(x) = x/2+2/x, x>0 .

Q 1.07 | Page 31

f(x) = (x+1) (x+2)1/3, $x\frac{>}{} - 2$ .

Q 1.08 | Page 31

f(x) = $x\sqrt{32 - x^2}, - 5\frac{<}{}x\frac{<}{}5$ .

Q 1.09 | Page 31

f(x) = $x^3 - 2a x^2 + a^2 x, a > 0, x \in R$ .

Q 1.1 | Page 31

f(x) = $x + \frac{a2}{x}, a > 0,$ , x ≠ 0 .

Q 1.11 | Page 31

f(x) = $x\sqrt{2 - x^2} - \sqrt{2} \leq x \leq \sqrt{2}$ .

Q 1.12 | Page 31

f(x) = $x + \sqrt{1 - x}, x \leq 1$ .

Q 2.1 | Page 31

f(x) = (x $-$ 1) (x $-$ 2)2.

Q 2.2 | Page 31

f(x) = $x\sqrt{1 - x} , x\frac{<}{}1$ .

Q 2.3 | Page 31

f(x) = $- (x - 1 )^3 (x + 1 )^2$ .

Q 3 | Page 31

The function y = a log x+bx2 + x has extreme values at x=1 and x=2. Find a and b ?

Q 4 | Page 31

Show that $\frac{\log x}{x}$ has a maximum value at x = e ?

Q 5 | Page 31

Find the maximum and minimum values of the function f(x) = $\frac{4}{x + 2} + x .$

Q 6 | Page 31

Find the maximum and minimum values of y = tan $x - 2x$ .

Q 7 | Page 31

If f(x) = x3 + ax2 + bx + c has a maximum at x = $-$ 1 and minimum at x = 3. Determine a, b and c ?

Q 8 | Page 31

Prove that f(x) = sinx + $\sqrt{3}$ cosx has maximum value at x = $\frac{\pi}{6}$ ?

#### Chapter 17: Increasing and Decreasing Functions solutions [Page 37]

Q 1.1 | Page 37

f(x) = 4x $-$ $\frac{x^2}{2}$ in [ $-$ 2,4,5] .

Q 1.2 | Page 37

f(x) = (x $-$ 1)2 + 3 in [ $-$ 3,1] ?

Q 1.3 | Page 37

f(x) = 3x4 $-$ 8x3 + 12x2$-$ 48x + 25 in [0,3] .

Q 1.4 | Page 37

f(x) = (x $-$ 2) $\sqrt{x - 1} \text { in }[1, 9]$ .

Q 2 | Page 37

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

Q 3 | Page 37

Find the absolute maximum and minimum values of the function of given by $f(x) = \cos^2 x + \sin x, x \in [0, \pi]$ .

Q 4 | Page 37

Find the absolute maximum and minimum values of a function f given by $f(x) = 12 x^{4/3} - 6 x^{1/3} , x \in [ - 1, 1]$ .

Q 5 | Page 37

Find the absolute maximum and minimum values of a function f given by $f(x) = 2 x^3 - 15 x^2 + 36x + 1 \text { on the interval } [1, 5]$ ?

#### Chapter 17: Increasing and Decreasing Functions solutions [Pages 72 - 74]

Q 1 | Page 72

Determine two positive numbers whose sum is 15 and the sum of whose squares is maximum.

Q 2 | Page 72

Divide 64 into two parts such that the sum of the cubes of two parts is minimum.

Q 3 | Page 72

How should we choose two numbers, each greater than or equal to $-$ 2, whose sum______________ so that the sum of the first and the cube of the second is minimum?

Q 4 | Page 72

Divide 15 into two parts such that the square of one multiplied with the cube of the other is minimum.

Q 5 | Page 72

Of all the closed cylindrical cans (right circular), which enclose a given volume of 100 cm3, which has the minimum surface area?

Q 6.1 | Page 72

A beam is supported at the two end and is uniformly loaded. The bending moment M at a distance x from one end is given by $M = \frac{WL}{2}x - \frac{W}{2} x^2$ .

Find the point at which M is maximum in case.

Q 6.2 | Page 72

A beam is supported at the two end and is uniformly loaded. The bending moment M at a distance x from one end is given by $M = \frac{Wx}{3}x - \frac{W}{3}\frac{x^3}{L^2}$ .

Find the point at which M is maximum in case.

Q 7 | Page 72

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 lengths of the two pieces so that the combined area of the circle and the square is minimum?

Q 8 | Page 72

A wire of length 20 m is to be cut into two pieces. One of the pieces will be bent into shape of a square and the other into shape of an equilateral triangle. Where the we should be cut so that the sum of the areas of the square and triangle is minimum?

Q 9 | Page 72

Given the sum of the perimeters of a square and a circle, show that the sum of there areas is least when one side of the square is equal to diameter of the circle.

Q 10 | Page 73

Find the largest possible area of a right angled triangle whose hypotenuse is 5 cm long.

Q 11 | Page 73

Two sides of a triangle have lengths 'a' and 'b' and the angle between them is $\theta$ What value of $\theta$ will maximize the area of the triangle? Find the maximum area of the triangle also.

Q 12 | Page 73

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 a box. What should be the side of the square to be cut off so that the volume of the box is maximum? Find this maximum volume.

Q 13 | Page 73

A rectangular sheet of tin 45 cm by 24 cm is to be made into a box without top, in cutting off squares from each corners 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 possible?

Q 14 | Page 73

A tank with rectangular base and rectangular sides, open at the top is to the constructed so that its depth is 2 m and volume is 8 m3. If building of tank cost 70 per square metre for the base and Rs 45 per square matre for sides, what is the cost of least expensive tank?

Q 15 | Page 73

A window in the form of a rectangle is surmounted by a semi-circular opening. The total perimeter of the window is 10 m. Find the dimension of the rectangular of the window to admit maximum light through the whole opening.

Q 16 | Page 73

A large window has the shape of a rectangle surmounted by an equilateral triangle. If the perimeter of the window is 12 metres find the dimensions of the rectangle will produce the largest area of the window.

Q 17 | Page 73

Show that the height of the cylinder of maximum volume that can be inscribed a sphere of radius R is $\frac{2R}{\sqrt{3}} .$

Q 18 | Page 73

A rectangle is inscribed in a semi-circle of radius r with one of its sides on diameter of semi-circle. Find the dimension of the rectangle so that its area is maximum. Find also the area ?

Q 19 | Page 73

Prove that a conical tent of given capacity will require the least amount of  canavas when the height is $\sqrt{2}$ times the radius of the base.

Q 20 | Page 73

Show that the cone of the greatest volume which can be inscribed in a given spher has an altitude equal to $\frac{2}{3}$ of the diameter of the sphere.

Q 21 | Page 73

Prove that the semi-vertical angle of the right circular cone of given volume and least curved surface is  $\cot^{- 1} \left( \sqrt{2} \right)$ .

Q 22 | Page 73

An isosceles triangle of vertical angle 2 $\theta$ is inscribed in a circle of radius a. Show that the area of the triangle is maximum when $\theta$ = $\frac{\pi}{6}$ .

Q 23 | Page 73

Prove that the least perimeter of an isosceles triangle in which a circle of radius r can be inscribed is $6\sqrt{3}$r.

Q 24 | Page 73

Find the dimensions of the rectangle of perimeter 36 cm which will sweep out a volume as large as possible when revolved about one of its sides ?

Q 25 | Page 73

Show that the height of the cone of maximum volume that can be inscribed in a sphere of radius 12 cm is 16 cm ?

Q 26 | Page 73

A closed cylinder has volume 2156 cm3. What will be the radius of its base so that its total surface area is minimum ?

Q 27 | Page 73

Show that the maximum volume of the cylinder which can be inscribed in a sphere of radius $5\sqrt{3 cm} \text { is }500 \pi {cm}^3 .$

Q 28 | Page 74

Show that among all positive numbers x and y with x2 + y2 =r2, the sum x+y is largest when x=y=r $\sqrt{2}$ .

Q 29 | Page 74

Determine the points on the curve x2 = 4y which are nearest to the point (0,5) ?

Q 30 | Page 74

Find the point on the curve y2=4x which is nearest to the point (2,$-$ 8).

Q 31 | Page 74

Find the point on the curve x2=8y which is nearest to the point (2,4) ?

Q 32 | Page 74

Find the point on the parabolas x2 = 2y which is closest to the point (0,5) ?

Q 33 | Page 74

Find the coordinates of a point on the parabola y=x2+7x + 2 which is closest to the strainght line y = 3x $-$ 3 ?

Q 34 | Page 74

Find the point on the curvey y2=2x which is at a minimum distance from the point (1,4).

Q 35 | Page 74

Find the maximum slope of the curve y= $- x^3 + 3 x^2 + 2x - 27 .$

Q 36 | Page 74

The total cost of producing x radio sets per  day is Rs $\left( \frac{x^2}{4} + 35x + 25 \right)$ and the price per set  at which they may be sold is Rs. $\left( 50 - \frac{x}{2} \right) .$ ind the daily output to maximum the total profit.

Q 37 | Page 74

Manufacturer can sell x items at a price of rupees $\left( 5 - \frac{x}{100} \right)$ each. The cost price is Rs  $\left( \frac{x}{5} + 500 \right) .$ Find the number of items he should sell to earn maximum profit.

Q 38 | Page 74

An open tank is to be constructed with a square base and vertical sides so as to contain a given quantity of water. Show that the expenses of lining with lead with be least, if depth is made half of width.

Q 39 | Page 74

A box of constant volume c is to be twice as long as it is wide. The material on the top and four sides cost three times as much per square metre as that in the bottom. What are the most economic dimensions?

Q 40 | Page 74

The sum of the surface areas of a sphere and a cube is given. Show that when the sum of their volumes is least, the diameter of the sphere is equal to the edge of the cube.

Q 41 | Page 74

A given quantity of metal is to be cast into a half cylinder with a rectangular base and semicircular ends. Show that in order that the total surface area may be minimum the ratio of the length of the cylinder to the diameter of its semi-circular ends is $\pi : (\pi + 2)$ .

Q 42 | Page 74

The strength of a beam varies as the product of its breadth and square of its depth. Find the dimensions of the strongest beam which can be cut from a circular log of radius a ?

Q 43 | Page 74

A straight line is drawn through a given point P(1,4). Determine the least value of the sum of the intercepts on the coordinate axes ?

Q 44 | Page 74

The total area of a page is 150 cm2. The combined width of the margin at the top and bottom is 3 cm and the side 2 cm. What must be the dimensions of the page in order that the area of the printed matter may be maximum?

Q 45 | Page 74

The space s described in time by a particle moving in a straight line is given by S = $t5 - 40 t^3 + 30 t^2 + 80t - 250 .$ Find the minimum value of acceleration.

Q 46 | Page 74

A particle is moving in a straight line such that its distance at any time t is given by  S = $\frac{t^4}{4} - 2 t^3 + 4 t^2 - 7 .$  Find when its velocity is maximum and acceleration minimum.

#### Chapter 17: Increasing and Decreasing Functions solutions [Page 80]

Q 1 | Page 80

Write necessary condition for a point x = c to be an extreme point of the function f(x).

Q 2 | Page 80

Write sufficient conditions for a point x=c to be a point of local maximum.

Q 3 | Page 80

If f(x) attains a local minimum at x=c, then write the values off' (c) and f'' (c).

Q 4 | Page 80

Write the minimum value of f(x) = $x + \frac{1}{x}, x > 0 .$

Q 5 | Page 80

Write the maximum value of f(x) = $x + \frac{1}{x}, x > 0 .$

Q 6 | Page 80

Write the point where f(x) = x log, x attains minimum value.

Q 7 | Page 80

Find the least value of f(x) =$ax + \frac{b}{x}$, where a>0, b>0 and x>0 .

Q 8 | Page 80

Write the minimum value of f(x) = xx .

Q 9 | Page 80

Write the maximum value of f(x) = x1/x.

Q 10 | Page 80

Write the maximum value of f(x) = $\frac{\log x}{x}$, if it exists .

#### Chapter 17: Increasing and Decreasing Functions solutions [Pages 80 - 82]

Q 1 | Page 80

The maximum value of x1/x, x>0 is

(a) e1/e

(b) $\left( \frac{1}{e} \right)^e$

(c) 1

(d) none of these

Q 2 | Page 81

If $ax + \frac{b}{x} \frac{>}{} c$ for all positive x where a,b,>0, then

(a) $ab < \frac{c^2}{4}$

(b) $ab \frac{>}{} \frac{c^2}{4}$

(c) $ab \frac{>}{} \frac{c^{}}{4}$

Q 3 | Page 81

The minimum value of $\frac{x}{\log_e x}$ is

(a) e
(b) 1/e
(c) 1
(d) none of these

Q 4 | Page 81

For the function f(x) = $x + \frac{1}{x}$

(a) x = 1 is a point of maximum
(b) x = $-$ 1 is a point of minimum
(c) maximum value > minimum value
(d) maximum value< minimum value

Q 5 | Page 81

Let f(x) = x3+3x$-$ 9x+2. Then, f(x) has
(a) a maximum at x = 1
(b) a minimum at x = 1
(c) neither a maximum nor a minimum at x = $-$ 3
(d) none of these

Q 6 | Page 81

The minimum value of f(x) = $x4 - x2 - 2x + 6$ is
(a) 6
(b) 4
(c) 8
(d) none of these

Q 7 | Page 81

The number which exceeds its square by the greatest possible quantity is

(a) $\frac{1}{2}$

(b) $\frac{1}{4}$

(c) $\frac{3}{4}$

(d) none of these

Q 8 | Page 81

Let f(x) = (x $-$ a)2 + (x $-$ b)2 + (x $-$ c)2. Then, f(x) has a minimum at x =

(a) $\frac{a + b + c}{3}$

(b) $\sqrt[3]{abc}$

(c) $\frac{3}{\frac{1}{a} + \frac{1}{b} + \frac{1}{c}}$

(d) none of these

Q 9 | Page 81

The sum of two non-zero numbers is 8, the minimum value of the sum of the reciprocals is

(a) $\frac{1}{4}$

(b) $\frac{1}{2}$

(c) $\frac{1}{8}$

(d) none of these

Q 10 | Page 81

The function f(x) = $\sum^5_{r = 1}$ (x $-$ r)2 assumes minimum value at x =

(a) 5
(b) $\frac{5}{2}$

(c) 3
(d) 2

Q 11 | Page 81

At x= $\frac{5\pi}{6}$ f(x) = 2 sin 3x + 3 cos 3x is

(a) 0
(b) maximum
(c) minimum
(d) none of these

Q 12 | Page 81

If x lies in the interval [0,1], then the least value of x2 + x + 1 is

(a) 3
(b) $\frac{3}{4}$

(c) 1
(d) none of these

Q 13 | Page 81

The least value of the function f(x) = $x3 - 18x2 + 96x$ in the interval [0,9] is

(a) 126
(b) 135
(c) 160
(d) 0

Q 14 | Page 81

The maximum value of f(x) = $\frac{x}{4 - x + x^2}$ on [ $-$ 1, 1] is

(a) $- \frac{1}{4}$

(b) $- \frac{1}{3}$

(c) $\frac{1}{6}$

(d) $\frac{1}{5}$

Q 15 | Page 81

The point on the curve y2 = 4x which is nearest to, the point (2,1) is

(a) $1, 2\sqrt{2}$

(b) (1,2)
(c) (1,$-$ 2)

(d) ( $-$ 2,1)

Q 16 | Page 82

If x+y=8, then the maximum value of xy is
(a) 8
(b) 16
(c) 20
(d) 24

Q 17 | Page 82

The least and greatest values of f(x) = x3$-$ 6x2+9x in [0,6], are

(a) 3,4
(b) 0,6
(c) 0,3
(d) 3,6

Q 18 | Page 82

f(x) = $\sin + \sqrt{3} \cos x$ is maximum when x =

(a) $\frac{\pi}{3}$

(b) $\frac{\pi}{4}$

(c) $\frac{\pi}{6}$

(d) 0

Q 19 | Page 82

If a cone of maximum volume is inscribed in a given sphere, then the ratio of the height of the cone to the diameter of the sphere is

(a) $\frac{3}{4}$

(b) $\frac{1}{3}$

(c) $\frac{1}{4}$

(d) $\frac{2}{3}$

Q 20 | Page 82

The minimum value of $\left( x^2 + \frac{250}{x} \right)$ is

(a) 75
(b) 50
(c) 25
(d) 55

Q 21 | Page 82

If(x) = x+$\frac{1}{x}$,x > 0, then its greatest value is

(a) $-$ 2

(b) 0
(c) 3
(d) none of these

Q 22 | Page 82

If(x) = $\frac{1}{4x2 + 2x + 1}$ then its maximum value is

(a) $\frac{4}{3}$

(b) $\frac{2}{3}$

(c) 1

(d) $\frac{3}{4}$

Q 23 | Page 82

Let x, y be two variables and x>0, xy=1, then minimum value of x+y is
(a) 1
(b) 2
(c) $2\frac{1}{2}$

(d) $3\frac{1}{3}$

Q 24 | Page 82

f(x) = 1+2 sin x+3 cos2x, $0\frac{<}{}x\frac{<}{}\frac{2\pi}{3}$ is

(a) Minimum at x =$\frac{\pi}{2}$

(b) Maximum at x = sin $- 1$ ( $\frac{1}{\sqrt{3}}$)

(c) Minimum at x = $\frac{\pi}{6}$

(d) Maximum at sin $- 1$ ($\frac{1}{6})$

Q 25 | Page 82

The function f(x) = $2 x^3 - 15 x^2 + 36x + 4$ is maximum at x =
(a) 3
(b) 0
(c) 4
(d) 2

Q 26 | Page 82

The maximum value of f(x) = $\frac{x}{4 + x + x^2}$ on [ $-$ 1,1] is

(a) $- \frac{1}{4}$

(b) $- \frac{1}{3}$

(c) $\frac{1}{6}$

(d) $\frac{1}{5}$

Q 27 | Page 82

Let f(x) = 2x3$-$ 3x2$-$ 12x + 5 on [ 2, 4]. The relative maximum occurs at x =

(a) $-$ 2

(b) $-$1
(c) 2
(d) 4

Q 28 | Page 82

The minimum value of x loge x is equal to
(a) e
(b) 1/e
(c) $-$ 1/e

(d) 2/e

(e) $-$ e

## RD Sharma solutions for Class 12 Mathematics chapter 17 - Increasing and Decreasing Functions

RD Sharma solutions for Class 12 Maths chapter 17 (Increasing and Decreasing Functions) include all questions with solution and detail explanation. This will clear students doubts about any question and improve application skills while preparing for board exams. The detailed, step-by-step solutions will help you understand the concepts better and clear your confusions, if any. Shaalaa.com has the CBSE Mathematics for Class 12 by R D Sharma (Set of 2 Volume) (2018-19 Session) solutions in a manner that help students grasp basic concepts better and faster.

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Concepts covered in Class 12 Mathematics chapter 17 Increasing and Decreasing Functions are Maximum and Minimum Values of a Function in a Closed Interval, Maxima and Minima, Simple Problems on Applications of Derivatives, Graph of Maxima and Minima, Approximations, Tangents and Normals, Increasing and Decreasing Functions, Rate of Change of Bodies Or Quantities, Introduction to Applications of Derivatives.

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