X dari cos(2x+60)cos(2x-60)=1/4

Lanjutan :

>>>>>
4x = 0 + k.360
x = 0 + k.90

k = 1, x = 90
k = 2, x = 180
k = 3, x = 270
k = 4, x = 360

HP = {90, 180, 270, 360}