Cosecant of Straight Angle

Theorem

 * $\csc 180^\circ = \csc \pi$ is undefined

where $\csc$ denotes cosecant.

Proof
From Cosecant is Reciprocal of Sine:
 * $\csc \theta = \dfrac 1 {\sin \theta}$

From Sine of Straight Angle:
 * $\sin \pi = 0$

Thus $\csc \theta$ is undefined at this value.