Study guide and 3 practice problems on:

Matrix inverses

$\newcommand{\bfA}{\mathbf{A}}$ $\newcommand{\bfB}{\mathbf{B}}$ $\newcommand{\bfC}{\mathbf{C}}$ $\newcommand{\bfF}{\mathbf{F}}$ $\newcommand{\bfI}{\mathbf{I}}$ $\newcommand{\bfa}{\mathbf{a}}$ $\newcommand{\bfb}{\mathbf{b}}$ $\newcommand{\bfc}{\mathbf{c}}$ $\newcommand{\bfd}{\mathbf{d}}$ $\newcommand{\bfe}{\mathbf{e}}$ $\newcommand{\bfi}{\mathbf{i}}$ $\newcommand{\bfj}{\mathbf{j}}$ $\newcommand{\bfk}{\mathbf{k}}$ $\newcommand{\bfn}{\mathbf{n}}$ $\newcommand{\bfr}{\mathbf{r}}$ $\newcommand{\bfu}{\mathbf{u}}$ $\newcommand{\bfv}{\mathbf{v}}$ $\newcommand{\bfw}{\mathbf{w}}$ $\newcommand{\bfx}{\mathbf{x}}$ $\newcommand{\bfy}{\mathbf{y}}$ $\newcommand{\bfz}{\mathbf{z}}$
The matrix $\bfB$ is the matrix inverse of $\bfA$ if $\bfA \bfB = \bfI$ and $\bfB \bfA = \bfI.$
The matrix inverse of $\bfA$ is denoted $\bfA^{-1}$.
The inverse of a $2\times2$ matrix is given by swapping the diagonal entries, negating the off-diagonal entries, and dividing by the determinant: $$\begin{pmatrix}a&b\\c&d\end{pmatrix}^{-1} = \frac{1}{ad-bc} \begin{pmatrix}d & -b \\ -c & a\end{pmatrix}$$
To find the inverse of a $3 \times 3$ matrix,
  1. Compute the minors of each element
  2. Negate every other element, according to a checkerboard pattern
  3. Take the transpose
  4. Divide by the determinant of the original matrix