（1+cos2a)/(tana/2-1/tana/2)
=2cos^2 a/{[(tana/2)^2-1]/tana/2}
=cos^2 a/{[(tana/2)^2-1]/(2tana/2)}
=cos^2 a/[1/(-tana)]
=-cos^2a *tana
=-cosasina
==1/2sin2a