Cotangent of Angle in Cartesian Plane

Theorem
Let $P = \tuple {x, y}$ be a point in the cartesian 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:
 * $\cot \theta = \dfrac x y$

where $\cot$ denotes the cotangent of $\theta$.