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## Re-using an equation

To repeat an existing equation, one wants not only to have the same mathematics in it, one also wants to re-use the original label it had. The amsmath package comes to our help, here:

\usepackage{amsmath}
...
$$a=b \label{eq1}$$
...
Remember that
$$a=b \tag{\ref{eq1}}$$


Here, the second instance of a=b will be typeset with a copy, made by the \tag command, of the label of the first instance.

amsmath.sty
Distributed as part of AMSLaTeX macros/latex/required/amslatex (or browse the directory); catalogue entry

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URL for this question: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=reuseq

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This is FAQ version 3.28, released on 2014-06-10.