这是利用重要极限lim(1+1/u)^u=e求出来的,变形如下lim(1+2c/x-c)^(x-c)=lim(1+2c/(x-c))^{[(x-c)/(2c)]2c}=lim{(1+2c/(x-c))^[(x-c)/(2c)]2c}=e^(2c).
