1．平方和公式n(n+1)(2n+1)/6
2．即1^2+2^2+3^2+…+n^2=n(n+1)(2n+1)/6 (注：N^2=N的平方)
3．证明1＋4＋9＋…＋n^2＝N(N+1)(2N+1)/6
