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$A = \left[ \begin{array}{{c}{c}{c}} a_{11} & a_{12} & a_{13} \\ a_{21} & a_{22} & a_{23} \\ a_{31} & a_{32} & a_{33} \\ \end{array} \right]$

$d e t (A) = a 11 a 22 a 33 + a 12 a 23 a 31 + a 13 a 21 a 32 - a 13 a 22 a 31 - a 11 a 23 a 32 - a 12 a 21 a 33$