jika 3 cos2 2x + 4 cos 2x - 4 = 0, maka cos x =

3 cos²2x + 4 cos 2x - 4 = 0
(3 cos 2x - 2)(cos 2x + 2) = 0
cos 2x = 2/3 atau cos 2x = -2
*namun karena nilai cos ada di interval -1≤cos≤1, maka cos 2x tdk memenuhi*

cos 2x = 2/3
2 cos²x - 1 = 2/3
2 cos²x = 5/3
cos²x = 5/6
cos x = ± √(5/6)