Cosine is Reciprocal of Secant

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

Then:
 * $\cos \theta = \dfrac 1 {\sec \theta}$

where $\cos$ denotes the cosine function and $\sec$ denotes the secant function.

Proof
$\sec \theta$ and $\dfrac {1} {\cos \theta}$ are not defined when $\cos \theta = 0$.

Also see

 * Trigonometric Functions in terms of each other