Determinant 2x2
Show the determinant of a 2x2 matrix
is
Proof
By definition of determinant, the determinant of a 2x2 matrix
is
where $\sigma$ is a one-to-one mapping from a set $\{1,2 \}$ to the same set.
There are two cases that
and
Let $\sigma_{12}$ be the above case and $\sigma_{21}$ be the below case.
$S_{2}$ is a set having $\sigma_{12}$ and $\sigma_{21}$ as elements,
$
\mathrm{sgn}(\sigma)
$
is a sign assigned to $\sigma_{12}$ and $\sigma_{21}$ as
($\mathrm{sgn}(\sigma)$ becomes negative if the order of number $\{1,2\}$ is changed odd times.)
Therefore
$|A|$ is written as
Example
Using the above result, the determinant of a 2x2 matrix
is
Calculator
Enter a 2x2 matrix and press "Execute" button. Then its determinant is displayed.
$|A|$
=