Cos is the expression of cosine function. 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 2π (k is an integer), the maximum value of the function is 1. 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.

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).

The square of either side of a triangle is equal to the sum of the squares of the other two sides minus the cosine of the included angle between the two sides, that is, in the cosine theorem, let C=90 degrees, then cosC=0, then c2=a2+b2.

(1) Given the length of three sides of a triangle, three internal angles can be found;

(2) Knowing the two sides and the included angle of the triangle, we can find the third side;

(3) Knowing the diagonal of two sides and one side of a triangle, we can find the other angles and the third side.