sina/2sin²a/2 ........

sina/2.sinˇ2 (a/2)=(sin 2.a/2)/2.sinˇ2(a/2)=2.sin(a/2)cos(a/2)/2.sinˇ2(a/2)=
=cos(aˇ2)/sin(a/2)=cotg(a/2)