## 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) = {x (x – 2)}2

Given:- Function f(x)= {x (x – 2)}2

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)= {x (x – 2)}2

f(x)= {[x2–2x]}2

f’(x)= 2(x2–2x)(2x–2)

f’(x)= 4x(x–2)(x–1)

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

f’(x) = 0

4x(x–2)(x–1)= 0

x(x–2)(x–1)= 0

x =0, 1, 2

Now, lets check values of f(x) between different ranges

Here points x = 0, 1, 2 divide the number line into disjoint intervals namely, (–, 0),(0, 1), (1, 2) and (2, ∞)

Lets consider interval (–, 0) and (1, 2)

In this case, we have x(x–2)(x–1)< 0

Therefore, f’(x) < 0 when x < 0 and 1< x < 2

Thus, f(x) is strictly decreasing on interval (–∞, 0)(1, 2)

Now, consider interval (0, 1) and (2, ∞)

In this case, we have x(x–2)(x–1)> 0

Therefore, f’(x) > 0 when 0< x < 1 and x < 2

Thus, f(x) is strictly increases on interval (0, 1)(2, ∞)

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