an=n^3-n 裂项相消求和~~~~~~~~~~~~~~~
问题描述：
an=n^3-n 裂项相消求和~~~~~~~~~~~~~~~
答
an=(n-1)n(n+1)=[(n+2)(n+1)n(n-1)-(n+1)n(n-1)(n-2)]/4
其前n项和为(n+2)(n+1)n(n-1)/4.