cofunction

A trig function paired with another via complementary angles, e.g. sine and cosine.

Two acute angles $\theta$ and $\varphi$ of a right triangle satisfy $\theta+\varphi=90°$ — they're complementary. Because "opposite" and "adjacent" swap when you switch which acute angle you stand at, each trig ratio equals its cofunction evaluated at the complementary angle: $\sin\theta=\cos(90°-\theta)$ , $\tan\theta=\cot(90°-\theta)$ , $\sec\theta=\csc(90°-\theta)$ .

Examples

$\sin 30° = \cos 60°$ , since $30°$ and $60°$ are complementary.