lim(x→正无穷）[(x^n)/(e^ax)] =lim(x→正无穷）[n*x^(n-1)]/[a*e^ax] =lim(x→正无穷）[n(n-1)*x^(n-2)]/[(a^2)*(e^ax)] =……………… =lim(x→正无穷）n!/[(a^n)*e^ax] =0
