Formula for the determinant of a 2-by-2 matrix.
Formula: For a $2 \times 2$ matrix:
$$\det\begin{pmatrix} a & b \\ c & d \end{pmatrix} = ad - bc$$
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Formula: For a $2 \times 2$ matrix:
$$\det\begin{pmatrix} a & b \\ c & d \end{pmatrix} = ad - bc$$