71

This question already has an answer here:

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

enter image description here

marked as duplicate by Werner, Paul Gaborit, T. Verron, barbara beeton, Mensch 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.

82

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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