Cos mathematical formula: COSA = (b 2+c 2-a 2)/2bc. Cosine theorem, the basic theorem of Euclidean plane geometry. Cosine theorem is a mathematical theorem describing the relationship between the length of three sides in a triangle and the cosine value of an angle. It is a generalization of Pythagorean theorem in general triangles, and Pythagorean theorem is a special case of cosine theorem. Cosine theorem is an important theorem to reveal the relationship between the angles of a triangle. It can be directly used to solve the third side of a triangle with known two sides and included angle or a triangle with known three sides. Cosine theorem will be more convenient and flexible to use if it is modified and properly transferred to other knowledge.