How to prevent the label of a long equation running into the right margin?

Question

I have an equation of the form

\begin{equation}\label{eq:master}
f(x) = E
\end{equation}

where E is a long expression. The problem is that the label runs into the right margin.

What is the proper way to fix this?

Answer 1

Use the amsmath package and then

\begin{multline}\label{eq:master}
f(x) =\\
 E
\end{multline}