令t=tan(x/2) t'=1/2*sec^2(x/2) 在y'=ln(tan(x/2))' =(lnt)'*t' =1/t×1/2*sec^2(x/2) =cot(x/2)×1/2*sec^2(x/2) =1/[2cos(x/2)sin(x/2)]