sin4a/(1+cos4a)*cos2a/(1+cos2a)*cosa/(1+cosa)*cos(a/2)/(1+cos(a/2))=
2sin2a*cos2a/(2cos2acos2a)*cos2a/(1+cos2a)*cosa/(1+cosa)*cos(a/2)/(1+cos(a/2))=sin2a/(1+cos2a)*cosa/(1+cosa)*cos(a/2)/(1+cos(a/2))=
以同法化简
=sin(a/2)/(1+cos(a/2))=tan(a/4)