(2sinX-sin2X)/(2sinX+sin2X)
=(2-2cosx)/(2+2cosx)
=(1-cosx)/(1+cosx);
tan^2(x/2)
=sin^2(x/2)/cos^2(x/2)
=[(1-cosx)/2]/[(1+cosx)/2]
=(1-cosx)/(1+cosx);
得证.