lim(x→0)[(1+x)/(1-x)]^(1/x)
=lim(x→0)[(1+2x/(1-x)]^[(1-x)/2x][2/(1-x)]
底数[(1+2x/(1-x)]^[(1-x)/2x]趋于e,指数[2/(1-x)]趋于2
极限=e^2