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