回複：再試牛人

using the fact (i+1)i(i-1)-i(i-1)(i-2)=3*i*(i-1)
n(n-1)+(n-1)(n-2)+...=(n+1)n(n-1)/3.........(1)
n+(n-1)+.....+1=(n+1)n/2.........(2)

(1)+(2)

所有跟帖： 

• 當然！ -botong- ♂ (34 bytes) () 07/22/2009 postreply 17:18:37

• I learnt the above method in E-school -說了就走- ♂ (215 bytes) () 07/22/2009 postreply 23:36:15