2 cos 3x + 1=0, 0°

2 Cos 3x + 1 = 0                              0 < x < 360
2 Cos 3x = -1
Cos 3x = -1/2
Cos 3x = cos 60°

Diketahui rumus Cos x = cos α, maka
x = α + k.360 dan x = -α + k.360, jadi

3x = 60 + k . 360
x = 20 + k . 120 (1)

dan

3x = 60 + k . 360
x = -20 + k . 120 (2)

(1) x = 20 + k . 120 =
k = 0 = 20 + 0 . 120 = 20°
k = 1 = 20 + 1 . 120 = 140°
k = 2 = 20 + 2 . 120 = 260°

(2) x = -20 + k .120 =
k = 1 = -20 + 1 . 120 = 100°
k = 2 = -20 + 2. 120 = 220°
k = 3 = -20 + 3 . 120 = 340°

Jadi, HP (20°,100°,140°, 220°, 260°, 340°)