сosx=(cosx/2-sinx/2)^2-1

cosx=cos²x/2+sin²x/2-2sinx/2cosx/2-1
cosx=1-sinx-1
cosx+sinx=0
sinx+sin(π/2-x)=0
2sinπ/4cos(x-π/4)=0
cos(x-π/4)=0
x-π/4=π/2+πn
x=π/4+π/2+πn
x=3π/4+πn