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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}

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3 Answers

up vote 20 down vote accepted

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}
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Thanks It works! –  Theta30 May 6 '11 at 1:38
10  
you could also simply put \nonumber at the end of each equation line for which you don't want an equation no. –  prettygully May 6 '11 at 1:44
    
You could also see this question: tex.stackexchange.com/questions/13396/… –  Ryan Reich May 6 '11 at 2:04
    
@prettygully: if the number of lines is even, then \notag (or \nonumber) won't vertically center the tag. –  Gonzalo Medina May 6 '11 at 2:06
    
@prettygully: I think you should also write your comment as an answer. @Gonzalo Medina: sometimes you want the number at the end, rather than centered; I often do so. (While the OP prefers centered, not everyone will.) –  Niel de Beaudrap May 6 '11 at 12:17
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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}
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I edited the code to remove syntax problems. –  Stefan Kottwitz May 6 '11 at 18:22
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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.

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Mildly ironic that in an answer to a question on alignment, you had an error in your code alignment! A case of misalliance (in tribute to Britain's answer to Tom Lehrer). –  Andrew Stacey May 6 '11 at 12:48
    
I think this is the right answer, but with the wrong example. To typeset a multi-line equation such as a = b = c = d = e, you should not use anything from the align family, but something like split. Could you perhaps change the example so that it is something like "a = b, c = d, e = f, g = h", so that it is clearly multiple equations that you want to align. This is not just semantics; there might be a difference in the vertical spacing, and you should use the right command for the right purpose. –  Jukka Suomela May 6 '11 at 12:59
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