Law Of Cosines

The law of cosnies extends the use of the Pythagorean theorem.  It enables us to find the length of the opposite side of a known angle when any enclosed angle is known.  It also enables us to find unknown angles when three sides are known.

It reads as follows:

a² = b² + c² - 2bc cos a

Similiarly, we may substitute sides and angles to complete the law to read:

b² = a² + c² - 2ac cos b

and
c² = a² + b - 2ab cos c