Cotangent is Cosine divided by Sine
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Theorem
Let $\theta$ be an angle such that $\sin \theta \ne 0$.
Then:
- $\cot \theta = \dfrac {\cos \theta} {\sin \theta}$
where $\cot$, $\sin$ and $\cos$ mean cotangent, sine and cosine respectively.
Proof
Let a point $P = \tuple {x, y}$ be placed in a cartesian plane with origin $O$ such that $OP$ forms an angle $\theta$ with the $x$-axis.
Then:
\(\ds \frac {\cos \theta} {\sin \theta}\) | \(=\) | \(\ds \frac {x / r} {y / r}\) | Cosine of Angle in Cartesian Plane and Sine of Angle in Cartesian Plane | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac x r \frac r y\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac x y\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \cot \theta\) | Cotangent of Angle in Cartesian Plane |
When $\sin \theta = 0$ the expression $\dfrac {\cos \theta} {\sin \theta}$ is not defined.
$\blacksquare$
Sources
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 5$: Trigonometric Functions: $5.16$
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): Appendix $12$: Trigonometric formulae
- 2021: Richard Earl and James Nicholson: The Concise Oxford Dictionary of Mathematics (6th ed.) ... (previous) ... (next): Appendix $14$: Trigonometric formulae