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I have the following question: I like to have a big curly brace behind the equations to show that they belong together and have only one equation number.

An example without the brace would be

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
\begin{split}
    x &= r\,\cos \varphi\\
    y &= r\,\sin \varphi
\end{split}
\end{align}

\end{document}

I would like to have the curly bracket behind the two equations. I saw this already in textbooks but have not found latex code for that. I guess it is very simple, but at the moment I have no clue how to do it. I was already searching for a while.

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Welcome to TeX.SE! –  Mico Jan 17 '12 at 1:41
    
Thank you very much! Very kind of you! –  Highspin Jan 17 '12 at 1:58
    
You're welcome! I've taken the liberty of fixing a typo in the first sentence and replacing "bracket" with "brace" in two instances, as "bracket" may be (mis)understood as a "square" bracket. –  Mico Jan 17 '12 at 2:07
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3 Answers

You could use the following, modified version of the code you've provided:

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{equation}
\left.
\begin{aligned}
    x &= r\,\cos \varphi \quad\\ % use \quad as spacer between equation and right brace
    y &= r\,\sin \varphi
\end{aligned}
\right\}
\end{equation}
\end{document}

enter image description here

By the way, both the align and split environments, which you employ in your code, let users set alignment points (e.g., the equal sign) with the & symbol. Using both environments simultaneously is redundant.

Note that I've substituted equation for align, and aligned for split, in the code above because you mention that you want the equations to be associated with a single equation number. (The align environment will create separate equation numbers for each equation it encounters unless you use the \notag command, but this seems more complicated than using the aligned environment inside an equation environment.)

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Thanks @Mico for the quick answer. But what do you mean with your comment that the align environment already provides the split functionality? How do I use it? Your construction for te bracket is not working in align, at least not how I used it :) –  Highspin Jan 17 '12 at 1:55
    
Sorry if I wasn't clear. What I meant to say is that the align and the split environments both let you set alignment points in the equation(s) -- frequently, but not necessarily so, the equal sign is chosen as the alignment point. Using both environments simultaneously is therefore redundant. If you want to retain the align environment, don't use the split environment as well. Finally, you had mentioned that you wanted a single equation number, so the align environment won't be correct in this setup as it'll create two equation numbers. –  Mico Jan 17 '12 at 2:08
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You could use the empheq package, which provides further ways of emphasizing equations:

\documentclass{article}
\usepackage{amsmath}
\usepackage{empheq}
\begin{document}
\begin{empheq}[right=\empheqrbrace]{equation}
  \begin{split}
    x &= r\,\cos \varphi\\
    y &= r\,\sin \varphi
  \end{split}
\end{empheq}
\end{document}

equations with brace

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Ah, much neater than numcases! –  percusse Jan 17 '12 at 7:38
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Another approach by the mathtools package.

\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{mathtools}

\begin{document}
  \begin{equation}
    \begin{rcases}
      x = r\,\cos\varphi\\
      y = r\,\sin\varphi
    \end{rcases}
  \end{equation}
\end{document}

You may also take a look at the cases package, which offers some more possibilities.

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