tga/2*tgb/2+tgb/2*tgc/2+tgc/2*tga/2
=tga/2*tgb/2+tgc/2*(tga/2+tgb/2)
=tga/2*tgb/2+tg[90-(a+b)/2]*(tga/2+tgb/2)
=tga/2*tgb/2+ct(a/2+b/2)*(tga/2+tgb/2)
=tga/2*tgb/2+(tga/2+tgb/2)/tg(a/2+b/2)
=tga/2*tgb/2+1-tga/2*tgb/2
=1