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