Pembahasan:
cos 2x + cos x = 0, 0° ≤ x ≤ 180°
Gunakan rumus: cos 2x = 2cos²x − 1
cos 2x + cos x = 0
2cos²x − 1 + cos x = 0
2cos²x + cos x − 1 = 0
(2 cosx − 1)(cos x + 1) = 0
2 cos x − 1 = 0 atau cos x + 1 = 0
2 cos x − 1 = 0 cos x + 1 = 0
cos x = ½ cos x = -1
cos x = cos 60 cos x = cos 180
x = 60 ± 360 x = 180 ± 360
= 60 = 180
Kaprikornus, himpunan penyelesaiannya 60°, 180°