cos2x+sin(x+3p/2)=0
cos^2x-sin^2x-cosx=0
2cos^2x-cosx-1=0
cosx=t
2t^2-t-1=0
D=9
x1=1
x2=-1/2
cosx=1 cosx=-1/2
x=2pik x=+-2pi/3+2pik
