Most geometry and pre-calculus books have entire units devoted to learning about two of the most important formulas in mathematics: the law of sines and the law of cosines. Both of these are very useful and very powerful tools, as they can be used to find unknown angles and sides of triangles. Yet, these books rarely devote anything to another formula that is equally powerful and useful: the law of tangents. The law of tangents can be used whenever two sides and one of the opposite angles are known. It can also be used when two angles and one of their opposite sides are known.