lim1/(1+e^1/x)
(x->0+) (1+e^1/x)->无穷大 lim1/(1+e^1/x)=0
(x->0-) (1+e^1/x)->-无穷大 lim1/(1+e^1/x)=0
所以lim1/(1+e^1/x) (x->0)=0