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I want to typeset the determinants of the system

$\begin{cases}
a_1x+ b_1y = c_1,\\
a_2x+ b_2y = c_2.
\end{cases}$

I tried

$D=\left| \begin{matrix}
   {{a}_{1}} & {{b}_{1}}  \\
   {{a}_{2}} & {{b}_{2}}  \\
\end{matrix} \right|={{a}_{1}}{{b}_{2}}-{{a}_{2}}{{b}_{1}}$

and

$D_x=\Biggl| \begin{matrix}
   {{c}_{1}} & {{b}_{1}}  \\
   {{c}_{2}} & {{b}_{2}}  \\
\end{matrix}  \Biggr|={{c}_{1}}{{b}_{2}}-{{c}_{2}}{{b}_{1}}$

I don't understand, I get enter image description here

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closed as unclear what you're asking by Qrrbrbirlbel, Werner, Claudio Fiandrino, Thorsten, lockstep Aug 19 '13 at 6:40

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question.If this question can be reworded to fit the rules in the help center, please edit the question.

2  
What's wrong with what you have? Please explain exactly what you're after. –  Werner Aug 19 '13 at 4:58

2 Answers 2

up vote 11 down vote accepted

I do not see a problem, except that the source can be simplified:

  • Many pairs of curly braces can be removed, e.g.: {a} is equivalent to \mathord{a} and a that is already an ordinary atom.
  • Package amsmath also provides environment vmatrix.

Example file:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\[
  D = \begin{vmatrix}
        a_{1} & b_{1}  \\
        a_{2} & b_{2}  \\
      \end{vmatrix}
  = a_{1} b_{2} - a_{2} b_{1}
\]
\end{document}

Result

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If you are asking for an environment suitable for determinants, there is vmatrix from amsmath package:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\(D_x = \begin{vmatrix}
   c_{1} & b_{1}  \\
   c_{2} & b_{2}
\end{vmatrix} = c_{1} b_{2} - c_{2} b_{1}\)


\end{document} 

enter image description here

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