（cota/2-tana/2)(1+tana tana/2)可化简为
问题描述：
（cota/2-tana/2)(1+tana tana/2)可化简为
答
(cot(a/2)-tan(a/2))(1+tan(a)*tan(a/2))
=(1/tan(a/2)-tan(a/2))(1+(2*(tan(a/2))^2)/(1-(tan(a/2))^2))
=((1-(tan(a/2))^2)/tan(a/2))((1-(tan(a/2))^2+2*(tan(a/2))^2)/(1-(tan(a/2))^2))
=(1+(tan(a/2))^2)/tan(a/2)
=cot(a/2)+tan(a/2)
答
答案是2csca