%%
%% http://tex.stackexchange.com/questions/298700/reproducing-gauss-famous-proof
%%
\documentclass{jarticle}
\usepackage{amsmath,booktabs,array}

\begin{document}

\begin{equation*}
\setlength{\arraycolsep}{0pt}
\begin{array}{ r *{5}{ >{{}}c<{{}} c } }
 S_n &=& a           &+& (a+d)      &+& \cdots &+& [a+(n-1)d]\\
\addlinespace
 S_n &=& [a+(n-1)d]  &+& [a+(n-2)d] &+& \cdots &+& a\\
\addlinespace
\midrule
\addlinespace
2S_n &=& [2a+(n-1)d] &+& [2a+(n-1)d &+& \cdots &+& [2a+(n-1)d]
\end{array}
\end{equation*}

\end{document}
