домножаем на 2: 2cos2x+2sin^2(x)-1=0
2cos2x-cos2x=0
cos2x=0
2x=pi/2+pi*n
x=pi/4+pi*n/2
x=-9pi/4; -11pi/4; -13pi/4
2cos2x+2sin^2(x)-1=0
2x=π/2+πn;n∈z;
x=π/4+πn/2;n∈z;
x=-9π/4; -11π/4; -13π/4.
