Show equation number only once in align environment

Question

I want the equation number to be shown only at the last line, or better, somewhere in the middle of all. So instead of (1), (2), ... only (1). I use \begin{align} ... \end{align}

Answer 1

You can use the split environment from the amsmath package:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
\begin{split}
  a &= b \\
  &=c \\
  &=d \\
  &=e
\end{split}
\end{align}

\end{document}

Answer 2

Use \nonumber to do so:

\begin{align}
  a & b \nonumber \\ % no number is shown
  c & d \\ % there is a number
  e & f \nonumber % no number
\end{align}

Answer 3

you can use the aligned environment from the amsmath package:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{gather}
\begin{split}
  m &=n \\
    &=o \\
    &=p \\
    &=q
\end{split}\\
\begin{aligned}
  a &=b + 1 \\
  c &=d \\
  e &=f \\
  g &=h
\end{aligned}
\end{gather}

\end{document}

this is generally recommended for straightforward expressions rather than embedding a sub-structure within align. see the user's manual for amsmath (amsldoc.pdf on ctan or type texdoc amsmath if you have a tex live installation).

edit: changed equation environment to gather to allow multiple sub-structures, and added an example of split as requested. since single letters on either side of the equals sign will give the illusion that everything is aligned, i lengthened one line to show that the alignments of the two sub-structures are really offset from one another. this result will get two equation numbers; either may be removed by use of \notag, but the positioning will not be optimal. sadly, i don't have a solution for that.