2n(n+2n+1)-2n(n+1) =2n(n+1)-2n(n+1) =2n(n+1)(n+1-n) =2n(n+1) 无论n是奇数还是偶数,n（n+1）都是偶数,就是2的倍数,再乘2一定是4的倍数.