Definition:Determinant/Matrix/Order 2

Definition
Let $\mathbf A = \left[{a}\right]_2$ be a square matrix of order $2$.

That is, let:
 * $\mathbf A = \begin{bmatrix}

a_{11} & a_{12} \\ a_{21} & a_{22} \end{bmatrix}$

Then the determinant of $\mathbf A$ is defined as:
 * $\begin{vmatrix}

a_{11} & a_{12} \\ a_{21} & a_{22} \end{vmatrix} = \operatorname{sgn} \left({1, 2}\right) a_{1 1} a_{2 2} + \operatorname{sgn} \left({2, 1}\right) a_{1 2} a_{2 1} = a_{1 1} a_{2 2} - a_{1 2} a_{2 1}$