How to put equation number only at the last step of a multi-line calculation?

Question

I have to write a calculation that takes two lines. I need to put number only at the last step. How do I do that? I used \nonumber at the end of the first line where I don't want to put equation number. But the number is being shown on the first line and not on the last line.

for example:

\begin{eqnarray}
\dot{\epsilon}&=& u_\mu u_\nu\Bigl(\dot{I}^{\mu\nu}_{(0)+}+\dot{I}^{\mu\nu}_{(0)-}\Bigr)\\
&=& u_\mu u_\nu\int\mathrm{dp}~p^\mu p^\nu\bigl(\dot{f}_{(0)}+\dot{\bar{f}}_{(0)})
\label{eq:1}
\end{eqnarray}

Answer 1

Don't use eqnarray ! It yields bad spacing. Replace it with one of the amsmath environments. Here you can use an aligned environment, with option [b], nested in equation:

\documentclass[11pt]{article}

\usepackage{amsmath}

\begin{document}

\begin{equation}
\begin{aligned}[b]
\dot{\epsilon}&= u_\mu u_\nu\Bigl(\dot{I}^{\mu\nu}_{(0)+}+\dot{I}^{\mu\nu}_{(0)-}\Bigr)\\
&= u_\mu u_\nu\int\mathrm{dp}~p^\mu p^\nu\bigl(\dot{f}_{(0)}+\dot{\bar{f}}_{(0)})
\end{aligned}
\label{eq:1}
\end{equation}

\end{document}

Answer 2

\nonumber at the end of the first line works. But be careful: At the end of the first line means before the line-breaking \\ that ends the first line.

MWE:

\documentclass{article}
\begin{document}
\begin{eqnarray}
\dot{\epsilon}&=& u_\mu u_\nu\Bigl(\dot{I}^{\mu\nu}_{(0)+}+\dot{I}^{\mu\nu}_{(0)-}\Bigr) \nonumber\\
&=& u_\mu u_\nu\int\mathrm{dp}~p^\mu p^\nu\bigl(\dot{f}_{(0)}+\dot{\bar{f}}_{(0)})
\label{eq:1}
\end{eqnarray}
\end{document}

Answer 3

You should never use eqnarray, the reasons are explained at eqnarray vs align and they should also be clear from a visual comparison. I also made some small changes, notably to the dimension of the parentheses and to the treatment of differentials.

\documentclass{article}
\usepackage[tbtags]{amsmath}

\newcommand\prediff[1]{\mathrm{d}#1\,}
\newcommand{\diff}{\mathop{}\!\mathrm{d}}

\begin{document}

This is not the best way to cope with the problem:
\begin{eqnarray}
\dot{\epsilon}&=& u_\mu u_\nu\Bigl(\dot{I}^{\mu\nu}_{(0)+}+\dot{I}^{\mu\nu}_{(0)-}\Bigr)\nonumber\\
&=& u_\mu u_\nu\int\mathrm{dp}~p^\mu p^\nu\bigl(\dot{f}_{(0)}+\dot{\bar{f}}_{(0)})
\label{not-the-best}
\end{eqnarray}
Perhaps you are better served with \texttt{split} (note
the option \texttt{tbtags}):
\begin{equation}\label{better}
\begin{split}
\dot{\epsilon}
&= u_\mu u_\nu\bigl(\dot{I}^{\mu\nu}_{(0)+}+\dot{I}^{\mu\nu}_{(0)-}\bigr)\\
&= u_\mu u_\nu\int\prediff p p^\mu p^\nu\bigl(\dot{f}_{(0)}+\dot{\bar{f}}_{(0)})
\end{split}
\end{equation}
With the differential at the end it would be
\begin{equation}\label{better-post}
\begin{split}
\dot{\epsilon}
&= u_\mu u_\nu\bigl(\dot{I}^{\mu\nu}_{(0)+}+\dot{I}^{\mu\nu}_{(0)-}\bigr)\\
&= u_\mu u_\nu\int p^\mu p^\nu\bigl(\dot{f}_{(0)}+\dot{\bar{f}}_{(0)})\diff p
\end{split}
\end{equation}

\end{document}