Law of Cosines

The Law of Cosines states that given a triangle with side lengths a, b, and c, and opposing angles A, B and C respectively:



Like the Law of Sines, this relationship also allows us to solve triangles that are not right triangles, such as triangle ABC below:

law of cosines


In order to use the Law of Cosines, the measure of the length of two sides and their included angle or the length of all three sides must be known.