[1+cos(a)]/2
=1/2+1/2cos(a/2+a/2)
=1/2+1/2[cos^2(a/2)-sin^2(a/2)]
=1/2-1/2sin^2(a/2)+1/2cos^2(a/2)
=1/2cos^2(a/2)+1/2cos^2(a/2)
=cos^2(a/2)
累死我了.