根据拉格朗日中值定理,lim(x→∞)(f(x)-f(x-1))=lim(x→∞)f'(w),x-1<=w<=x,该式=e.
lim((x+c)/(x-c))^x=lim((1+c/x)/(1-c/x))^x=(lim(1+c/x)^x) ÷(lim(1-c/x)^x)=e^c/e^(-c)=e^(2c)
故c=1/2