jika 3 cos² 2x + 4 cos 2x - 4 = 0, nilai cos x adalah...

cos 2x = y
3 y² + 4 y - 4 = 0
(3y- 2)(y+2) = 0
y = 2/3  ∨ y= -2
cos  2x = 2/3  ∨ cos 2x = -2 (Tidak Memenuhi, krn -1≤ cos ax ≤ 1

cos 2x = 2/3
2cos² x - 1 = 2/3
2 cos² x = 1 + 2/3 = 5/3
cos² x = 5/6
cos x = +_ √(5/6)
cos x = +- 1/6 √30