ln(1+x)-ln(1-x)=ln[(1+x)/(1-x)]=ln[1+2x/(1-x)]
x→0,等价无穷小代换
ln[1+2x/(1-x)] ~2x/(1-x)
lim(x→0) [ln(1+x)-ln(1-x)]/x
=lim(x→0) 2x/(1-x)x
=2