cos 2x + cos 4x + cos 6x = 0
cos 4x + (cos 2x + cos 6x) = 0
cos 4x + (2 cos 4x cos 2x) = 0
(1 + 2 cos 2x) cos 4x = 0
(1 + 2 cos 2x) = 0
cos 2x = -½
2x = 120°
x = 60°
sau
2x = 240°
x = 120°
sau
cos 4x = 0
4x = 90°
x = 22.5°
sau
4x = 270°
x = 67.5°
x = 22.5° , 60° , 67.5° , 120°
