nX(n+1)= n^2+n所以原式等=（1+2+3+...+n）+（1+2^2+3^2+4^2+...+n^2
原式 = \sum_{i=1}^{n}(i^2 + i) = \sum_{i=1}^{n}{i^2} + \sum_{i=1}^{n}{i} = n(n+1)(2n+1)/6 + n(n+1)/2
