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

f(x) = x^{4} – 4x

Given:- Function f(x) = x^{4} – 4x

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^{4} – 4x

⇒

⇒ f’(x) = 4x^{3} – 4

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

⇒ f’(x) = 0

⇒ 4x^{3} – 4 = 0

⇒ 4(x^{3} – 1) = 0

⇒ x = 1

clearly, f’(x) > 0 if x > 1

and f’(x) < 0 if x < 1

Thus, f(x) increases on (1, ∞)

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

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