Cos4x-cos2x=1. Указать корни на отрезке [П\2;2П]

2cos^22x-cos2x-2=0   cos2x=(1+-3)/4 cos2x=1                2x=2Пl   x=Пl   (П ; 2П) cos2x=-1/2            2х=П+-П/3+2Пk                                             x=П/2+-П/6+Пk                                             x= 2П/3                                              x=5П/3