n*[e^2-(1+1/n)^2n]=n*(1+1/n)^2n*[e^2/(1+1/n)^2n-1]~e^2*n*ln[e^2/(1+1/n)^2n](等价无穷小因子替换)=e^2*n*[2-2n*ln(1+1/n)]=e^2*[2/n-2*ln(1+1/n)]/(1/n^2)再转化成连续函数求极限:令x(x趋于0+)~1/n(n趋于正无穷)lime^2*[2x-2*ln(1+x)]/x^2用一次罗比达结果为e^2则原极限=e^2