tolong jawab no 4 ya plz

sin^2 x - 2 cos x - 1 = 0
sin^2 x - 2 cos x - (sin^2 x + cos^2 x) = 0
-cos^2 x - 2 cos x = 0
- cos x ( cos x + 2) = 0
cos x = 0, maka x = π/2 atau 3π/2