ln|y|=ln|x|+1/2[1/(1-x)-1/(1+x)]
两边求导:y'/y=1/x+1/2[1/(x-1)-1/(1+x)]=1/x+1/(x^2-1)=-(x^2+x-1)/√[(1-x)(1+x)^3)]
(-1<x<1)