[1-tan^2(a/2)]/tan(a/2) *[1+2tan^2(a/2)]/[1-tan^2(a/2)]
=[1-tan^2(a/2)+2tan^2(a/2)]/tan(a/2)
=[1+tan^2(a/2)]/tan(a/2)
=1/sin(a/2)cos(a/2)=2/sina