Suppose we have a triangle with sides a,b and c and that the angle between b and c is ninety degrees. The Pythagorean Theorem states that a square is equal to b square plus c square. But, if there is no ninety degree angle in that triangle, we cannot use the Pythagorean theorem. However we CAN use the Law of Cosines.
The Law of Cosines states that a square is equal to b square plus c square MINUS two time b times c times the cosine of the angle between b and c, regardless whether the angle is ninety degrees or not.
In this video, I show you a couple of examples of the use of the Law of Cosines.