23

Suppose I have the equations

\begin{align}
 f(x) = x^2
 g(x) = \exp( x )
\end{align}

I would like to use on single label for these lines, and ideally the equation number would be placed vertically between these two lines.

Although this question appears very basic, I have not found an explanation neither via search engines, nor on this website.

marked as duplicate by mafp, Stefan Kottwitz Mar 4 '13 at 14:58

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

28

The very thing you want is the split environment, provided by the amsmath class.

Sample code.

\documentclass{article}
\usepackage[paperwidth=90mm]{geometry}% <-- better demonstrates the vertical alignment
\usepackage{amsmath}
\begin{document}

Eqn.~\eqref{eqn:eqlabel} has a single label split
across the two equations, as you can see here:
\begin{align}
\label{eqn:eqlabel}
\begin{split}
 f(x) &= x^2 ,
\\
 g(x) &= \exp( x ) .
\end{split}
\end{align}

\end{document}

Result.

Sample document using the split environment

  • Note: You can also use split not just within the align environment, but also within eqnarray. – Paul Nov 28 '14 at 21:07
  • 1
    That's as maybe, but you should not use eqnarray. – Niel de Beaudrap Nov 29 '14 at 17:15
  • forgot double slash \\ : f(x) &= x^2 ,\\ g(x) &= \exp( x ) . – Ivan Kush Dec 20 '16 at 11:31
  • How to align equ no. (1) with last line? – Mr.EU Mar 31 '17 at 17:49
  • @Mr.EU: in that case you should use equation together with aligned, as in \begin{equation} \label{eqn:eqlabel} \begin{aligned}[b] f(x) &= x^2 , \\ g(x) &= \exp( x ) . \end{aligned} \end{equation}. Note the [b] after the {aligned}: this puts the equation number on the bottom line (more specifically, it aligns the baseline of the bottom of the entire aligned environment with the baseline of whatever comes before and after it in the equation, such as the equation number). – Niel de Beaudrap Apr 6 '17 at 9:11
13

You can use an aligned or gathered environment inside an equation:

Sample output

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation}
  \label{eq:t}
  \begin{aligned}
    f(x) &= x^2\\        
    g(x) &= \exp(x)
  \end{aligned}
\end{equation}

\begin{equation}
  \label{eq:u}
  \begin{gathered}
    f(x) = x^2\\        
    g(x) = \exp(x)
  \end{gathered}
\end{equation}

\end{document}
  • forgot double slash \\ : f(x) = x^2\\ g(x) = \exp( x ) – Ivan Kush Dec 20 '16 at 11:31
  • @IvanKush Thanks for pointing it out. I am 100% certain I didn't forget them, because the code was cut and pasted from the file used to compile the image. However, they had certainly disappeared from the posting. – Andrew Swann Dec 20 '16 at 15:11

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