It's a bit ironic, but the author repeated the same error every time they wrote the formula involving sigma. What they wanted to write was $\sum_{0}^{n} k$. Wha

It's a bit ironic, but the author repeated the same error every time they wrote the formula involving sigma. What they wanted to write was $\sum_{0}^{n} k$. What they actually wrote ($\sum_{0}^{n} n$) is equal to $n \cdot (n+1)$ (without the division by 2).