## 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,

For f(x) to be increasing, we must have

f’(x) > 0

(x – 2) > 0

2 < x < ∞

x (2, )

Thus f(x) is increasing on interval (2, )

Again, For f(x) to be decreasing, we must have

f’(x) < 0

(x – 2) < 0

–∞ < x < 2

x (–∞, 2)

Thus f(x) is decreasing on interval (–∞, 2)

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