Space Vector Angles
Why
We generalize the notion of angles in the plane to angles in space.
Definition
The angle
(unsigned angle) between
$x \in \R ^3$ and $y \in \R ^3$ is the real
number
\[
\theta = \angle(x, y) = \cos^{-1}\frac{x^\top
y}{\norm{x}\norm{y}}
\]