Geometry · Similarity & Right Triangles
Cosine
HSG-SRT.C.8
Definition
In a right triangle, the ratio of the side adjacent to an angle to the hypotenuse.
Examples
- cos(theta) = adjacent over hypotenuse (CAH).
- If the adjacent side is 4 and the hypotenuse is 5, cos(theta) = 4/5 = 0.8.
Key rule
Cosine always compares the adjacent side to the hypotenuse.
Memory hook
CAH — Cosine is Adjacent over Hypotenuse.