Cosine theorem of Cosa formula: cosC=(a2+b2-c2)/ab, cosB=(a2+c2-b2)/ac, cosA=(c2+b2-a2)/bc. The cosine theorem of triangle is an important theorem to reveal the relationship between angles of triangle. It can be directly used to solve the problem of finding the included angle of the third side or three known sides of a triangle. The ratio of the adjacent side to the hypotenuse of the acute angle of a right triangle is called the cosine of the acute angle.

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.