To find the length of a side of a triangle using cosine, you first need to find the angle and which sides are given. After you determine the angle and given sides, you'll need to figure out which trigonometric ratio to use. If you are given the adjacent side and the hypotenuse, you need to use cosine to solve. The degree equals adjacent divided by hypotenuse. In the example, Cos 39=3.5/x. After simplifying the equation, we found x to be 4.49. If the directions ask you to round, be sure to round your answers. 4.49 rounds to 4.5.