Question

1X2+2X3+3X4+...+nX(n+1)= 1X2+2X3+3X4+...+10X11= （写过程,如1/3 乘.

Answer 1

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