The cosine formula of vector included angle is: cos = (the inner product of AB) /(| A ||| B |). The included angle formula is a basic mathematical formula, which is divided into tangent formula and complementary angle formula. The tangent formula is expressed by tan, and the complementary angle formula is expressed by COS. Tangent formula (slope formula of straight line): k=(y2-y 1)/(x2-x 1), cosine formula (slope formula of straight line): k=(y2-y 1)/(x2-x 1)