# Cotangent of Angle in Cartesian Plane

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## Theorem

Let $P = \tuple {x, y}$ 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:

- $\cot \theta = \dfrac x y$

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

## Proof

## Sources

- 1968: Murray R. Spiegel:
*Mathematical Handbook of Formulas and Tables*... (previous) ... (next): $\S 5$: Trigonometric Functions: $5.10$