Tangent of Complement equals Cotangent

Theorem

 * $\tan \left({\dfrac \pi 2 - \theta}\right) = \cot \theta$ for $\theta \ne n \pi$

where $\tan$ and $\cot$ are tangent and cotangent respectively.

That is, the cotangent of an angle is the tangent of its complement.

This relation is defined wherever $\sin \theta \ne 0$.

Proof
The above is valid only where $\sin \theta \ne 0$, as otherwise $\dfrac {\cos \theta} {\sin \theta}$ is undefined.

From Sine of Multiple of Pi it follows that this happens when $\theta \ne n \pi$.