(sin (3x/2)+ cos(3x\2))^2=1\2
sin^2(3x/2)+2sin3x/2cos3x/2+cos^2(3x/2)=1/2
1+sin3x=1/2
sin3x=-1/2
3x=(-1)^(n+1)*pi/6+pi n
x=(-1)^(n+1)*pi/18+1/3pi n
