We use the law of Cosines to be able to find : 1. The measure of the third side, when the measure of two sides and the included angle of a triangle ABC are known. 2. The measure of any angle, when the measure of the three sides of a triangle ABC are known.
The Law of Cosines is c² = a² + b² - 2abcosΘ. It has four variables; a, b, c and Θ. Whenever you have a triangle with three of these variables known, use the Law of Cosines to calculate the unknown variable.