|z|^2=(1+cosa)^2+(sina)^2
=1+1+2cosa
=2+2cosa
=2(1+cosa)
=2*2cos^2 (a/2)
=4cos^2 (a/2)
因为π＜α＜2π,所以π/2＜α/2＜π,cos(α/2)