lim f(x) =lim 1/[1+e^(1/x)]=1/1=1;
//x→0-,则1/x→-∞;则e^(1/x)→0.
lim f(x) =lim 1/[1+e^(1/x)]=1/(+∞)=0;
//x→0+,则1/x→+∞;则e^(1/x)→+∞
∴
只有右连续;