Tangent is Reciprocal of Cotangent

Theorem
Let $\theta$ be an angle such that $\sin \theta \ne 0$ and $\cos \theta \ne 0$.

Then:
 * $\tan \theta = \dfrac 1 {\cot \theta}$

where $\tan$ denotes the tangent function and $\cot$ denotes the cotangent function.

Proof
$\tan \theta$ is not defined when $\cos \theta = 0$, and $\cot \theta$ is not defined when $\sin \theta = 0$.

Also see

 * Trigonometric Functions in terms of each other