cos^2x-5cos2x=2sin^2x

(1+cos2x)/2 -5cos2x-2(1-cos2x)/2=0
1+cos2x-10cos2x-2+2cos2x=0
-7cos2x=1
cos2x=-1/7
2x=π-arccos1/7+2πn
x=π/2 -1/2arccos1/7+πn