Determinant of $$$\left[\begin{array}{cc}5 - \lambda & -4\\-4 & 5 - \lambda\end{array}\right]$$$

Calculate $$$\left|\begin{array}{cc}5 - \lambda & -4\\-4 & 5 - \lambda\end{array}\right|$$$.

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

$$$\left|\begin{array}{cc}5 - \lambda & -4\\-4 & 5 - \lambda\end{array}\right| = \left(5 - \lambda\right)\cdot \left(5 - \lambda\right) - \left(-4\right)\cdot \left(-4\right) = \lambda^{2} - 10 \lambda + 9$$$

$$$\left|\begin{array}{cc}5 - \lambda & -4\\-4 & 5 - \lambda\end{array}\right| = \left(\lambda - 9\right) \left(\lambda - 1\right)$$$A