Butuh secepatnya nomor 28

jawabannya adalah a.
cos 2x - 2 cos x = -1
2cos² x - 1 - 2 cos x + 1 = 0
2cos² x - 2 cos x = 0
2 cos x (cos x - 1) = 0
cos x = 0 atau cos x = 1

cos x = 0 = cos π/2 
x = π/2 + k. 2π , dg k = 0,1,2,3
k = 0, x = π/2
k = 1, x = 3π/2

cos x = 1 = cos 0
x= 0 + k.2π  dgn k = 0,1,2,...
k = 0 , x = 0
k = 1 , x = 2π

x = (0, 1/2π, 3/2π, 2π)