ABCDEFGHIJKLMNOPQRSTUVWXYZ Definition of Law of Cosines more ... For any triangle, the Law of Cosines says: c^{2} = a^{2} + b^{2} − 2ab cos(C) Where: • a, b and c are the three sides of the triangle, and • C is the angle opposite side c It helps us solve some triangles. The Law of Cosines