2cos^2 x/4-2sin^2 x/4=-1 решить уравнение

2cos^2 x/4 -2sin^2x/4= -1,
2(cos^2 x/4 - sin^2 x/4)= -1,
(cos^2 x/4 - sin^2 x/4) = -1/2,
cos(2(x/4))= -1/2, cos(x/2)= -1/2,
x/2=+-2π/3+2πk, k∈Z, 
x=+-4π/3 +4πk, k∈Z