# Cosine

Cosine is a trigonometric function describing the ratio of the adjacent side to the hypotenuse in a right triangle. It is often abbreviated cos.

x values y values
0 1
π/2 0
π -1
3π/2 0
1

radians is equivalent to one circle. The graph continues in that pattern.

## Relation to secant

$\cos \theta = \sin \left(\frac{\pi}{2} - \theta \right) = \frac{1}{\sec \theta}\,$