## Book: RD Sharma - Mathematics (Volume 1)

### Chapter: 17. Increasing and Decreasing Functions

#### Subject: Maths - Class 12th

##### Q. No. 1 of Exercise 17.2

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##### Find the intervals in which the following functions are increasing or decreasing.

Given:- Function

Theorem:- Let f be a differentiable real function defined on an open interval (a,b).

(i) If f’(x) > 0 for all , then f(x) is increasing on (a, b)

(ii) If f’(x) < 0 for all , then f(x) is decreasing on (a, b)

Algorithm:-

(i) Obtain the function and put it equal to f(x)

(ii) Find f’(x)

(iii) Put f’(x) > 0 and solve this inequation.

For the value of x obtained in (ii) f(x) is increasing and for remaining points in its domain it is decreasing.

Here we have,

f’(x) = x3 + 2x2 – 5x – 6

For f(x) lets find critical point, we must have

f’(x) = 0

x3 + 2x2 – 5x – 6 = 0

(x+1)(x – 2)(x + 3) = 0

x = –1, 2 , –3

clearly, f’(x) > 0 if –3 < x < –1 and x > 2

and f’(x) < 0 if x < –3 and –3 < x < –1

Thus, f(x) increases on (–3, –1) (2, ∞)

and f(x) is decreasing on interval (∞, –3) (–1, 2)

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