y=((1-cosx/2)/2+(1+cosx/2)/2)^2-2((1-cosx/2)/2)((1+cosx/2)/2)=(1)^2-1/2 * (1-cosx/2)(1+cosx/2)=1-1/2 * (1-cos^2(x/2))=1-1/2+1/2 * (1+cosx)/2=1/2+1/4+(1/4)cosx=3/4+(1/4)cosx