### 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}
>     ...
>     \begin{equation}
>       a=b
>       \label{eq1}
>     \end{equation}
>     ...
>     Remember that
>     \begin{equation}
>       a=b
>       \tag{\ref{eq1}}
>     \end{equation}

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.

Comprehensive documentation of AMSLaTeX is to be found in amsldoc, in the distribution; it is also available on the web at ftp://ftp.ams.org/pub/tex/doc/amsmath/amsldoc.pdf

`amsldoc.tex`
  Distributed as part of AMSLaTeX macros/latex/required/amslatex (zip, browse)
`amsmath.sty`
  Distributed as part of AMSLaTeX macros/latex/required/amslatex (zip, browse)

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=reuseq