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How is it possible to make a brace and have something like this in a eqnarray block?

enter image description here

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marked as duplicate by Werner, Paul Gaborit, T. Verron, barbara beeton, Kurt Jul 6 '13 at 18:01

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.

    
Welcome to TeX.SX! You don't have to sign with your name since it automatically appears in the lower right corner of your post. (But you should know this? ;) Could you also add a minimal working example (MWE)?) – Sean Allred Jul 6 '13 at 16:18
2  
See \eqnarray vs \align on why not to use eqnarray. Then Large braces for specifying values of variables by condition as a possible duplicate, which suggests using cases from amsmath. – Werner Jul 6 '13 at 16:29
up vote 23 down vote accepted

Here's a sample document without the use of amsmath package:

\documentclass{article}
\begin{document}
  \[
    \alpha(x)=\left\{
                \begin{array}{ll}
                  x\\
                  \frac{1}{1+e^{-kx}}\\
                  \frac{e^x-e^{-x}}{e^x+e^{-x}}
                \end{array}
              \right.
  \]
\end{document} 

or one with amsmath's cases environment

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
  \alpha(x)=\begin{cases}
               x\\
               \frac{1}{1+e^{-kx}}\\
               \frac{e^x-e^{-x}}{e^x+e^{-x}}
            \end{cases}
\]
\end{document} 
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