Cos x + cos^2x=1/2-sin^2x

cosx+cos²x=1/2-sin²x cosx+cos²x+sin²x=1/2 cosx+1=1/2 cosx=-1/2 a)x=2pi/3+2k.pi b)x=4pi/3+2k.pi k=0,1,-1,2,-2,.......