先用
ln根号
(1+5x)
=ln[(1+5x)^(1/2)]
=(1/2)ln(1+5x)
然后当x是小量时
ln(1+x)~x
等价小量替换
lim x->0 ln根号(1+5x)/x
=lim x->0 (1/2)ln(1+5x)/x
=lim x->0 (1/2)*5x/x
=5/2
