Determinant_of_Diagonal_Matrix

Let $\mathbf A = \begin{bmatrix} a_{11} & 0 & \cdots & 0 \\ 0 & a_{22} & \cdots & 0 \\ \vdots & \vdots & \ddots & \vdots \\ 0 & 0 & \cdots & a_{nn} \\ \end{bmatrix}$ be a diagonal matrix. Then the determinant of $\mathbf A$ is the product of the elements of $\mathbf A$. That is $\ds \map \det {\mathbf A} = \prod_{i \mathop = 1}^n a_{ii}$