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I'm trying to get the equation shown in the picture. But i only manage to get the following coding. Can anyone help me to label it with numbering like the picture? I tried with \begin{eqnarray} or \begin{equation} but end up with many errors.

\[ \left\{
  \begin{array}{lr}
    \dot {x}_1 = f_1(x_1,\dots,x_n) \\
     \hspace{13pt}   \dots \\
    \dot {x}_n = f_n(x_1,\dots,x_n)  \\
  \end{array}
\right.
\]

HELP

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Welcome to TeX.SX! You can format TeX code by selecting it and clicking on the {} button (or adding four spaces in front of each line). Change \[ and \] into \begin{equation} and \end{equation} respectively. –  egreg Dec 12 '13 at 10:22
    
@Andrew Ooi - Please see the startup guide ;) –  Kiyoshi Dec 12 '13 at 18:52
    
@egreg Thanks alot. :D –  Andrew Ooi Dec 13 '13 at 3:21

2 Answers 2

up vote 9 down vote accepted

Instead of inserting a row that starts with three horizontal dots, you could also create a row with a set of vertical dots centered on the = signs. The code below implements this idea via the \vdotswithin macro of the mathtools package.

enter image description here

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
    \begin{cases} % \! %% uncomment "\!" instruction if you want the equations
                       %% to be snugged up a bit against the left-hand curly brace
       \begin{aligned}
          \dot {x}_1 &= f_1(x_1,\dots,x_n) \\
          &\vdotswithin{=} \\  % vertical dots, centered on the "=" signts
          \dot {x}_n &= f_n(x_1,\dots,x_n)
       \end{aligned} 
    \end{cases}
\end{equation}
\end{document}
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Don't forget \! before \begin{aligned}. –  stalking is prohibited Dec 12 '13 at 11:47
2  
@DonutE.Knot - Thanks for this suggestion. I've added a comment in the code to suggest the insertion of \! in case the OP wants to "snug up" the equations a bit against the left-hand curly brace. –  Mico Dec 12 '13 at 12:10
    
@DonutE.Knot In this case, it's not as necessary as in other cases, since aligned is not used inside one logical equation (where you need \!), rather it's used as it should be: to align multiple equations inside other math and the \{ is not part of the equations. Therefore I would advocate here for keeping the extra space. –  yo' Dec 12 '13 at 12:15
    
@tohecz - thanks. I was thinking along the same lines, and that's why I made the \! optional... –  Mico Dec 12 '13 at 12:50

Try with this code:

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

\begin{equation}
    \begin{cases}
        \dot {x}_1 = f_1(x_1,\dots,x_n) \\
        \ldots \\
        \dot {x}_n = f_n(x_1,\dots,x_n)
    \end{cases}
\end{equation}

\end{document}

You can also use your code, but with some changes (which egreg said):

\begin{equation}
    \begin{cases}
        \begin{array}{lr}
            \dot {x}_1 = f_1(x_1,\dots,x_n) \\
            \hspace{13pt}   \dots \\
            \dot {x}_n = f_n(x_1,\dots,x_n)  \\
        \end{array}
    \end{cases}
\end{equation}
share|improve this answer
3  
equation should be used instead of align. –  egreg Dec 12 '13 at 10:22
    
@egreg - Ok, I will change. –  Kiyoshi Dec 12 '13 at 10:24

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