Plane Vector Angles
Why
We talk about angles in the plane using the language of vectors.
Definition
The angle
(unsigned angle) between
$x \in \R ^2$ and $y \in \R ^2$ is the real
number
\[
\theta = \angle(x, y) = \cos^{-1}\frac{x^\top
y}{\norm{x}\norm{y}}
\]