 ## 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.f(x) = 6 – 9x – x2

Given:- Function f(x) = 6 – 9x – x2

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) = 6 – 9x – x2 f’(x) = –9 – 2x

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

f’(x) > 0

–9 – 2x > 0

–2x > 9   Thus f(x) is increasing on interval Again, For f(x) to be decreasing, we must have

f’(x) < 0

–9 – 2x < 0

–2x < 9   Thus f(x) is decreasing on interval 1
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