Find the x-coordinates where f '(x) = 0

Question

Find the x-coordinates where f '(x) = 0 for f(x) = 2x + sin(2x) in the interval [0, 2π]

Answer 1

f'(x)=2+2 cos (2x)

f'(x)=0 when cos(2x) = -1, i.e. when x pi/2 and 3pi/2