(1+cos2a)/(tana/2)-(cota/2)
=(1+cos2a)*(cota/2)-(cota/2)
=cos2acota/2
是不是打错了
(1+cos2A)/[tan(A/2)-cot(A/2)]
=2cosA^2/[sin(A/2)/cos(A/2)-cos(A/2)/sin(A/2)]
=2cosA^2*sin(A/2)*cos(A/2)/[sin(A/2)^2-cos(A/2)^2]
=cosA^2*sinA/(-cosA)
=-cosA*sinA
=-sin(2A）/2