令t=1/x,t 趋向 0
(cost) ^ (1/t^2)=[1+(cost-1)] ^ [1/(cost-1) * (cost-1)/t^2] =e^ [(cost-1)/t^2]
又 cost-1在t趋向0 的时候 等价 -1/2 t^2
所以原式=e^(-1/2 )