[sin2a/(1+cos2a)][cosa/(1+cosa)] 最后一个-是加+
={[2sinacosa]/2(cosa)^2}[cosa/(1+cosa)]
=(sina/cosa)[cosa/(1+cosa)]
=sina/(1+cosa)
=[2sin(a/2)cos(a/2)]/2[cos(a/2)]^2
=tan(a/2)
另有：tana/2=sina/(1+cosa)=(1-cosa)/sina
证：sina/(1+cosa)=[2sin(a/2)cos(a/2)]/cos²(a/2)
=tan(a/2)(1-cosa)/sina=sin²(a/2)/[2sin(a/2)cos(a/2)]=tan(a/2)
所以,原式=cot(a/2)