=(1+cosa)/(1-cosa)
[tan(a/2)]^2其中
tan(a/2)=sin(a/2)/cos(a/2)上下乘cos(a/2)
得cos(a/2)sin(a/2)/[cos(a/2)]^2=sina/2[cos(a/2)]^2=sina/(cosa-1)
则[tan(a/2)]^2=(sina)^2/(cosa-1)^2=(1-cosa)(1+cosa)/(1-cosa)^2
=(1+cosa)/(1-cosa)