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).

Extended data:

The domain of cosine function is a whole set of real numbers, and the range of values is [- 1, 1]. It is a periodic function with a minimum positive period of 2π. When the independent variable is 2kπ(k is an integer), the maximum value of the function is1; When the independent variable is (2k+ 1)π, the function has a minimum value of-1. Cosine function is an even function, and its image is symmetrical about y axis.

Using cosine theorem, we can solve the following two kinds of problems about triangles:

(1) Given three sides, find three angles;

(2) Knowing the angles between two sides and them, find the third side and the other two angles.