Secant of Straight Angle

Theorem

 * $\sec 180^\circ = \sec \pi = -1$

where $\sec$ denotes secant.

Also see

 * Sine of Straight Angle
 * Cosine of Straight Angle
 * Tangent of Straight Angle
 * Cotangent of Straight Angle
 * Cosecant of Straight Angle