{1+sina-2[sin(45°-a/2)]^2}÷[4cos(a/2)]
=﹛1+sina-[1-cos(90°-a﹚]﹜／[4cos(a/2)]
=[1+sina-﹙1-sina﹚]／[4cos(a/2)]
=2sina／[4cos(a/2)]
=4sin(a/2)cos(a/2)／[4cos(a/2)]
=sin(a/2﹚