设k=lim(x->0)[ (e^x+x)^(1/x)]
lnk =lim(x->0) [ln(e^x+x) /x] ("0/0"型,用洛必达法则分子分母同时求导)
= lim(x->0)(e^x+1)/(e^x+x)
=2
所以k= e^2