复合求导
[f(g(x))]=f'(g(x))*g'(x)
所以这里f(x)=ln x,g(x)=x^2+1
所以f'(x)=1/x
f'(g(x))=1/(x^2+1)
g'(x)=2x
所以dy=(1/(x^2+1)*2x)dx=2xdx/(x^2+1)