Tolong jawab no 4 yaplz

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