Cos 2x+ cos x = 0 untuk 0 < x < π

cos2x=2cos²x-1
2cos²x+cosx-1=0
(2cosx-1)(cosx+1)
2cosx=1
cosx=1/2
x= arccos (1/2) = 60°,300°
cosx=-1
x= arccos (-1) = 180°
HP={60°}