由定义:(lnx)'
=lim(h->0) [ln(x+h)-lnx]/h
=lim(h->0) ln[(1+h/x)^(1/h)]
=lim(h->0) ln{[(1+1/(x/h))^(x/h)]^(1/x)}
=ln[e^(1/x)]
=1/x