I am trying to make use of the array environment (anything that operates similarly would suffice) that will automatically stretch the columns to be even-width and spread across the entire page. One example:
\documentclass[12pt]{article}
\usepackage{amsmath}
\renewcommand{\u}[1]{\underline{#1}}
\begin{document}
\[\begin{array}{rrr}
5 & 5 & 5\\
\u{+4} &\u{+4} &\u{+4}
\end{array}\]
\end{document}
All that I know how to do at the moment is to insert a 'fake' column between each pair of columns and use \hspace*{##cm} to force the columns to spread out, but this gets a bit tedious if I have, say 7 columns on 13 rows that, for one reason or another, cannot all be a part of a single array environment.
Is there a semi-simple approach to this problem? I've been searching, but I haven't found anything that has seemed very helpful (it may be the case that I'm not familiar enough with the jargon to enter the best search phrases, too).

\newcolumntype{R}{>{$}r<{$}}in the preamble and then typebegin{tabular*}{\linewidth}{R@{\extracolsep{\fill}}RR}in the body of the document. – Mico Aug 01 '15 at 02:15tabular*method, when used with the@{\extracolsep{\fill}}device, will equalize the amounts of intercolumn whitespace, while the widths of the columns themselves will depend on what's in the columns. If, instead, you want the columns themselves to occupy equal widths, you should probably use atabularxenvironment instead. – Mico Aug 01 '15 at 02:32tabular*environment to0.8\linewidthand encase it in acenterenvironment. – Mico Aug 01 '15 at 02:42