(1+tan(a/2)/(1-tan(a/2)) 分子,分母都乘以cos(a/2)
=(cosa/2+sina/2)/(cosa/2-sina/2)
=(cosa/2+sina/2)^2/(cosa/2-sina/2)(cosa/2+sina/2)
=(1+2sina/2cosa/2)/(cosa/2^2-sina/2^2)
=(1+sina)/cosa,得证.