Study guide and
2 practice problems
on:
The method of diagonals for computing the determinant of a 3x3 matrix
$\newcommand{\bfA}{\mathbf{A}}$ $\newcommand{\bfB}{\mathbf{B}}$ $\newcommand{\bfC}{\mathbf{C}}$ $\newcommand{\bfF}{\mathbf{F}}$ $\newcommand{\bfI}{\mathbf{I}}$ $\newcommand{\bfa}{\mathbf{a}}$ $\newcommand{\bfb}{\mathbf{b}}$ $\newcommand{\bfc}{\mathbf{c}}$ $\newcommand{\bfd}{\mathbf{d}}$ $\newcommand{\bfe}{\mathbf{e}}$ $\newcommand{\bfi}{\mathbf{i}}$ $\newcommand{\bfj}{\mathbf{j}}$ $\newcommand{\bfk}{\mathbf{k}}$ $\newcommand{\bfn}{\mathbf{n}}$ $\newcommand{\bfr}{\mathbf{r}}$ $\newcommand{\bfu}{\mathbf{u}}$ $\newcommand{\bfv}{\mathbf{v}}$ $\newcommand{\bfw}{\mathbf{w}}$ $\newcommand{\bfx}{\mathbf{x}}$ $\newcommand{\bfy}{\mathbf{y}}$ $\newcommand{\bfz}{\mathbf{z}}$
The determinant of a $3 \times 3$ matrix can be computing by adding the products of terms on the forward diagonals and subtracting the products of terms on the backward diagonals.
The forward diagonals are given as
The backward diagonals are given as
Using these, $$ \text{det} \begin{pmatrix} a&b&c\\d&e&f\\g&h&i \end{pmatrix} = aei + bfg + cdh - afh - bdi - ceg.$$
Related topics
Determinant of a 3x3 matrix
(15 problems)
Determinants
(23 problems)
Multivariable calculus
(147 problems)
Practice problems
Use the method of diagonals to compute the determinant $$\text{det } \begin{pmatrix} 0 & 1 & 2 \\ 3 & -1 & 0 \\ 1 & -2 & 1 \end{pmatrix}$$
Solution
Compute the determinant of $$\begin{pmatrix}1 & -1& 1 \\ 1 & 1 & 1\\1 & 2 & 4 \end{pmatrix}$$
Solution
