trigonometri, kurang ngerti bantuin ya kak

Penting: cos 2x = 2 cos² x -1 

cos 2x + 3 cos x +2 =0
(2 cos² x - 1 ) + 3 cos x +2 =0 
2 cos² x + 3 cos x + 1 = 0
(cos x + 1 ) ( 2 cos x + 1) = 0
cos x = -1 
x= π

2 cos x = -1
cos x = -0.5
x= 2π/3 dan 4π/3