Cos 2x-2cosx=-1,0<2Π

cos 2x - 2cos x = - 1
( 2os^2 x -1) - 2 cos x +1 = 0
2cos^2x - 2 cos x = 0
2cos x ( cos x - 1 ) = 0
2cos x = 0
cos x = 0
cos x = cos 90, 270
cos x - 1 = 0
cos x = 1
cos x = cos 0, 360
Jadi HP = { 0°, 90°, 270°, 360°}