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This question already has an answer here:

Whenever an expression is too long, it will be broken automatically in inline mode but done nothing in displaymode

E.g.

MWE:

\documentclass[11pt]{article}
\begin{document}

$1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0$

\[1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0\]

 \end{document}

The upper one will be broken whereas the lower one not.

So how to make auto break also works in displaymode?

marked as duplicate by Martin Schröder, Kurt, lockstep, Claudio Fiandrino, Werner Mar 10 '13 at 18:56

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