The adjacent side (adjacent short side) of an angle in a triangle is longer than the hypotenuse (longest side). Cosine (cosine function), a kind of trigonometric function. In Rt△ABC (right triangle), ∠ c = 90 (as shown in the figure), and the cosine of ∠A is the hypotenuse of the adjacent side of the triangle, that is, cosA=b/c, which can also be written as cosa=AC/AB. Cosine function: f(x)=cosx(x∈R).

Cosine theorem of extended data, 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.

The history of cosine theorem can be traced back to Euclid's Elements of Geometry before the third century BC. In the book, triangles are divided into obtuse angles and acute angles, which correspond to the positive and negative cosine values in modern mathematics respectively.

Reference cosine _ Baidu Encyclopedia