f'-(0)=lim[x→0-] [f(x)-f(0)]/x
=lim[x→0-] [x/(1+e^(1/x))] / x
=lim[x→0-] 1/(1+e^(1/x))
=1
注：x→0-时,1/x→-∞,e^(1/x)→0
f'+(0)=lim[x→0+] [f(x)-f(0)]/x
=lim[x→0+] [x/(1+e^(1/x))] / x
=lim[x→0+] 1/(1+e^(1/x))
=0
注：x→0+时,1/x→+∞,e^(1/x)→+∞
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