% !TEX program = pdfLaTeX
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\documentclass[preview]{standalone}
\input{00_myPreamble.tex}
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\begin{document}
$\displaystyle
  \mathrm{det}(J) = 
  \begin{vmatrix}
    a & b \\
    c & d 
  \end{vmatrix} =
  ad-bc
$
\end{document}