I need help with this question:

f(x) = sin(2x) + cos(2x) where x is an element of [0,pie].

Find the interval on which this function is increasing and decreasing.

I know that I must calculate f'(x) > 0 and f'(x) < 0

But I'm not good at differentiating trigonometric functions and at finding the intervals.

I would very grateful for any help.