Cosine theorem formula is cosA=(b? +c？ -a？ ) /2bc, cosA= adjacent side is more than hypotenuse. 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 can be used to solve the problem of the third side of a triangle with two sides and included angles known or the angles of three sides known.

Cosine theorem is an important theorem for solving triangles, which can be applied to the following three requirements:

1. When two sides of a triangle and their included angles are known, the opposite sides of the known angles can be found from the cosine theorem.

2. When the three sides of a triangle are known, the three internal angles of the triangle can be obtained by cosine theorem.

3. When the three sides of a triangle are known, the area of the triangle can be obtained by cosine theorem.