# pmatrix line break

I wrote a matrix using the pmatrix environment. The content is very long and LaTeX produces an output of the kind (the "a" line extending beyond \textwidth):

( aaa aaa aaa aaa aaa aaa + aaa aaa aaa )
( bb bb bb bb bb bb bb bb               )
( cccccccccccccccccc                    )


I would like to get:

( aaa aaa aaa aaa aaa aaa )
(           + aaa aaa aaa ) [aligned right]
( bb bb bb bb bb bb bb bb )
( cccccccccccccccccc      )


\documentclass{article}
\usepackage{amsmath}
\begin{document}
$\begin{pmatrix} aaa aaa aaa aaa aaa aaa + aaa aaa aaa \\ bb bb bb bb bb bb bb bb\\ cccccccccccccccccc \end{pmatrix}$
\end{document}

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Welcome to TeX.SE! I doubt if any matrix environment is adequate for your expression, which appears to have 9 entries in the first row, 8 in the second, and only 1 in the third. (Usually, matrices are rectangular arrays with an equal number of entries in each row.) –  Mico Sep 8 '12 at 18:22

The only problem with simply adding an extra line break is the alignment, which you can fix by adding a \hfill.
\documentclass{article}
$\begin{pmatrix} aaa aaa aaa aaa aaa aaa \\ \hfill + aaa aaa aaa \\ bb bb bb bb bb bb bb bb\\ cccccccccccccccccc \end{pmatrix}$

I'd add {} in front of + in order to get correct spacing. –  egreg Sep 8 '12 at 19:16