cos(-3x)=1/2,untuk 0 < x < 360

cos (-x) = cos x
cos (-3x) = 1/2
cos 3x = cos 60°

3X1 = 60°+k.360°
X1 = 20°+k.120°
k=0->x=20°
k=1->x=140°
k=2->x=260°

3X2 = -60°+k.360°
X2 = -20°+k.120°
k=1->x=100°
k=2->x=220°
k=3->x=340°

HP={20°,100°,140°,220°,260°,340°}