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