sinx * sin2x=1-cos²x
sinx * 2sinx*cosx=sin²x
2 * sin²x* cosx-sin²x=0
sin²x(2cosx-1)=0
sin²x=0, sinx=0, x=πn, n∈Z
2cosx-1=0, cosx=1/2, x=±arccos1/2+2πk, x=±π/3+2πk, k∈Z
