2 sin^2 - cos^2 -1 =0

2sin^2x-(1-sin^2x)-1=0
2sin^2x-1+sin^2x-1=0
3sin^2x-2=0
3sin^2x=2
sin^2x=2/3
sinx=2/3
x=(-1)^k•arcsin(2/3)+пn
sinx=(-2/3)
x=(-1)^k+1•arcsin(2/3)+пn