# 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 amslatex