令1/a=-2/x
x=-2a
所以原式=lim(a→∞)(1+1/a)^(-2a-2)
=lim(a→∞)1/(1+1/a)^(2a+2)
=lim(a→∞)1/[(1+1/a)^a]^2*(1+1/a)^2
=1/e^2