C（n,3）=n!/[(n-3)!* 3!]
=(n-2)*(n-1)*n/6
C（n,3）/(n^3 + 1）=(n-2)*(n-1)*n/[6*(n^3 + 1)]
因为lim[6*(n^3 + 1)]= 6 ≠ 0
所以 lim｛(n-2)*(n-1)*n/[6*(n^3 + 1)]｝
= lim[（n-2)*(n-1)*n]/lim[6*(n^3 + 1)]
=0/6
=0