The triangle cos formula has cosA=(b? +c？ -a？ )/2bc，cosB=(a？ +c？ -B? )/2ac，cosC=(b？ +a？ -c？ ) /2ab. The trigonometric cosine formula is mainly applied to the cosine theorem, that is, the square of any one side is equal to the sum of the squares of the other two sides minus the product of the cosine of the angle formed by these two sides.

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. 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.