令u=tanx
可化为 (1/1+u)d arctanu=1/(1+u)(1+u^2)du=1/2[(1/u^2+1-u/u^2+1 )+(1/u+1)]du
=1/2arctanu-1/4ln(u^2+1)+1/2ln(u+1)+C
再把u代入即可