The following might be in line with what you're after:

\documentclass{article}
\usepackage{amsmath,array}% http://ctan.org/pkg/{amsmath,array}
\newcolumntype{C}{>{{}}c<{{}}}
\begin{document}
\begin{align*}
x = 1 + 2 + 3 + 4 + \cdots + n \\
x = n + (n-1) + (n-2) + (n-3)+\cdots+ 1\\
\cline{1-2}
2x = (1+n) + (1+n) + (1+n) + (1+n) + \cdots (1+n)
\end{align*}
\[
\renewcommand{\arraystretch}{1.2}
\begin{array}{r*{12}{@{}C}}
x & = & 1 & + & 2 & + & 3 & + & 4 & + & \cdots & + & n \\
x & = & n & + & (n-1) & + & (n-2) & + & (n-3) & + & \cdots & + & 1 \\
\hline
2x & = & (1 + n) & + & (1 + n) & + & (1 + n) & + & (1 + n) & + & \cdots & + & (1+n)
\end{array}
\]
\end{document}
The entire structure is set in an array
with a column type that provides the appropriate spacing around binary operators and relations (by inserting an empty group {}
on the left/right of every cell, thanks to the array
package's \newcolumntype
). Some extra padding on the left and right (default of \arraycolsep
or 5pt) can be changed, if needed.
$
here.align*
on itself starts math-mode. If you want to usealign*
(oralign
), you need to use&
as an alignment point, in your case probably right before the=
signs. – Qrrbrbirlbel Oct 13 '13 at 4:32