# align* but show one equation number at the end

I am using align*, but I still need an equation number. I know one solution is to use align and add \nonumber to all lines but the last. Is there a 'lazy' way to do it?

I searched a little and found similar ones suggesting use split: Show equation number only once in align environment

Unfortunately, due to the page break issue I have, I need to use align, because split won't auto page break, as described in Long equation does not page break.

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Use \tag:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
a &=b \\
&=c \numberthis \label{eqn}
\end{align*}
Equation \eqref{eqn} shows that $a=c$.
$$d = e$$
\end{document}

See page 3 of the amsmath package documentation for details.

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 this \numberthis works beautifully. Thank you very much! – cpp initiator Jan 29 '12 at 22:16

Here are minimal working examples that I use. This is what I have as a header

\documentclass[12pt]{amsart}

\usepackage{amsmath,amsfonts,amssymb,epsfig}
\usepackage{graphicx}
\usepackage{hyperref}
%\usepackage{srcltx}
\usepackage{color}

Two equations each with separate equation numbers and separate reference labels

\begin{align}
\dot{a} &  =2\alpha(t)\eta\label{classicalmotion1}\\
\dot{\eta} &  =-2\gamma(t)a\label{classicalmotion2}%
\end{align}%

Two equations with single equation number and single reference label

\begin{aligned} \dot{A} & =2i\alpha(t)B\\ \dot{B} & =2i\gamma(t)A \end{aligned} \label{eqn2.qo}

I am guessing you know how to do the one without equation numbers.

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