I am writing an elementary summation proof and would like to write that
1 + 2 + 3 + ... + n-2 + n-1 + n
+ n + n-1 + n-2 + ... + 3 + 2 + 1
= n+1 + n+1 + n+1 + ...
and so on. I want the symbols to line up so that it's easy to see the vertical addition. How do I do this in LaTeX? The align environment doesn't do so well with multiple things to align on, and creating a tabular environment is annoying because it's not in math mode.
My LaTeX is a little rusty, but here goes:
\[
\begin{array}{l r c r c r c c c r c r c r}
& 1 & + & 2 & + & 3 & + & \ldots & + & n-2 & + & n-1 & + & n \\
+ & n & + & n-1 & + & n-2 & + & \ldots & + & 3 & + & 2 & + & 1 \\
= & n+1 & + & n+1 & + & n+1 & + & \ldots & + & n+1 & + & n+1 & + & n+1\\
\end{array}
\]
The array environment is the way to go here. This solution is better in some ways than Jack Kelly's, and worse in other ways:
\begin{array}{c *{6}{c@{\hspace{6pt} + \hspace{6pt}}} c}
& 1 & 2 & 3 & \ldots & n-2 & n-1 & n \\
+ & n & n-1 & n-2 & \ldots & 3 & 2 & 1 \\
= & n+1 & n+1 & n+1 & \ldots & n+1 & n+1 & n+1
\end{array}
Two things to notice in the column specification: we use *{6}{spec} to get six copies of a column with specification spec. We also use @ to specify that each column separator should look like a six-point gap, a plus sign, and another gap.
More on the @ trick here.