Question

Which of the following functions are strictly decreasing on?

(A) cos x (B) cos 2x (C) cos 3x (D) tan x

Answer 1

(A) Let

In interval

is strictly decreasing in interval.

(B) Let

is strictly decreasing in interval.

(C) Let

The point divides the intervalinto two disjoint intervals

i.e., 0

∴ f₃ is strictly increasing in interval.

Hence, f₃ is neither increasing nor decreasing in interval.

(D) Let

In interval

∴ f₄ is strictly increasing in interval

Therefore, functions cos x and cos 2x are strictly decreasing in

Hence, the correct answers are A and B.