CosA=(b2+c2-a2)/2bc. 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, and it is a generalization of Pythagorean theorem in the case of a general triangle. The formula of cosine theorem is cosA=(b2+c2-a2)/2bc, where A is the inner angle of a triangle, and A, B and C are three sides of the triangle respectively. This formula can be used to calculate any internal angle of a triangle when other values of internal angle and side length are known.