Determinant of $\left[\begin{array}{cc}1 & 2\\7 & 8\end{array}\right]$

Calculate $\left|\begin{array}{cc}1 & 2\\7 & 8\end{array}\right|$.

The determinant of a 2x2 matrix is $\left|\begin{array}{cc}a & b\\c & d\end{array}\right| = a d - b c$.

$\left|\begin{array}{cc}1 & 2\\7 & 8\end{array}\right| = \left(1\right)\cdot \left(8\right) - \left(2\right)\cdot \left(7\right) = -6$

$\left|\begin{array}{cc}1 & 2\\7 & 8\end{array}\right| = -6$A