Q: Let f(x)=20x-10sin(4x) for 0<=x<=pi/2

Find the largest interval(s) over which the function f is increasing or decreasing.

I tried to solve this:

f'(x)=20-40cos(4x)

=20(1-2cos(4x))

on keeping f'(x)=0, i get x=pi/12

I just cannot get correct intervals to work properly with my online math homework system!

Please help me!

Thank you SO much!