f(x)=1/(1-cos x)=1/(2(sin(x/2)^2)根据（cot x)'=1/sin(x) ^2,则有∫f(x) dx=cot(x/2)+C
1/(1-cos x)=1/(2(sin(x/2)^2)根据（cot x)'=1/sin(x) ^2,则有1/(1-cos x)=1/2 (cot (x/2))
利用半角的去做
