Secant of Angle in Cartesian Plane

Theorem
Let $P = \left({x, y}\right)$ be a point in the cartesian coordinate plane whose origin is at $O$.

Let $\theta$ be the angle between the $x$-axis and the line $OP$.

Let $r$ be the length of $OP$.

Then:
 * $\sec \theta = \dfrac r x$

where $\sec$ denotes the secant of $\theta$.