Q:

LaTeX align stacked summation

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.

A:

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.

