jika cos x = 1/2 maka cos 3x =

Anggap daerah penyelesaiannya 0° ≤ x ≤ 360°

cos x = 1/2
cos x = cos 60°
x = 60° ± k.360°
x = 60° + 0.360°
x = {60°}
dan
x = -60° ± k.360°
x = -60° + 1.360°
x = 300°
x = {300°}

Jadi, x = {60°, 300°}

Jika x = 60°
cos 3x = cos 180° = -1

Jika x = 300°
cos 3x = cos 900°
= cos (900°-720°)
= cos 180°
= -1

Jadi, cos 3x = -1