f(x) = ½sin²x - cos2x
f'(x) = ½ 2 sinx (sinx)' + sin2x (2x)' = sinxcosx + 4sinxcosx = 5sinxcosx
5sinxcosx = 0
sinx = 0
x = πn, n∈Z
cosx = 0
x = π/2 + πn, n∈Z
