2
I have this
\begin{array}{r}
sum &= 1 + 2 + \ldots + (n - 1) + n \\[1ex]
+ \; sum &= n + (n-1) + \ldots + 2 + 1 \\
\hline
2\,sum &= (n +1) + (n+1) + \ldots + (n+1) + (n+1)
\end{array}
for displaying the Gauss trick for summing integers proof, but the alignment is not great. Any sugge...
