Source TeX:

 \sum_{i=0}^{n-1}\left(n^3 + in + \frac{1}{2}n^2 + \frac{1}{2}   n     \right) = \sum_{i=0}^{n-1} \left(n^3+\frac{1}{2}n^2 + \frac{1}{2}n\right) + \sum_{i=0}^{n-1}in