f（x）=e^2x-2x
f＇（x）=2e^2x-2
f＇（x）除以e^x-1=2e^2x-2/(e^x-1)
=2[(e^x+1)(e^x-1)/(e^x-1)]
=2(e^x+1)
x趋近于0时
2(e^x+1)=2(1+1)=4