align* but show one equation number at the end

Question

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.

Answer 1

Use \tag:

\documentclass{article}
\usepackage{amsmath}
\newcommand\numberthis{\addtocounter{equation}{1}\tag{\theequation}}
\begin{document}
\begin{align*}
a &=b \\
  &=c \numberthis \label{eqn}
\end{align*}
Equation \eqref{eqn} shows that $a=c$.
\begin{equation}
d = e
\end{equation}
\end{document}

See page 3 of the amsmath package documentation for details.

Answer 2

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

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