lim(x→0)[1+f(x)/x]^(1/x)=lim(x→0)[1+f(x)/x]^[(x/f(x))*f(x)/x^2 ]
lim(x→0)f(x)/x^2=lim(x→0)f'(x)/2x=lim(x→0)f''(x)/2=2
所以 lim(x→0)[1+f(x)/x]^(1/x)=e^2
lim(x→0)[1+f(x)/x]^(1/x)=lim(x→0)[1+f(x)/x]^[(x/f(x))*f(x)/x^2 ]
lim(x→0)f(x)/x^2=lim(x→0)f'(x)/2x=lim(x→0)f''(x)/2=2
所以 lim(x→0)[1+f(x)/x]^(1/x)=e^2