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.

Also see

 * Trigonometric Functions in terms of each other