# Cosine Law

## Cosine Law

In any triangle, a relationship that makes it possible to establish that the square of one side is equal to the sum of the squares of the two other sides, minus two times the product of these sides by the cosine of the angle that forms them.

In the triangle ABC below, the cosine law takes these three forms:

• $${a^2} = {b^2} + {c^2} – 2bc \cos α$$
• $${b^2} = {a^2} + {c^2} – 2ac \cos β$$
• $${c^2} = {a^2} + {b^2} – 2ab \cos δ$$