【证明3|n(n+1)(2n+1)】
n(n+1)(2n+1) => n(n+1)(n+2+n-1) => n(n+1)(n+2) + n(n+1)(n-1)
因为n(n+1)(n+2)、n(n+1)(n-1)是连续的3个整数，故：
3|n(n+1)(n+2) & 3|n(n+1)(n-1) ＝》3|n(n+1)(2n+1)
