What does cos represent in trigonometric terms?

In trigonometric terms, cos (cosine) represents the ratio of the length of the adjacent side to the hypotenuse in a right triangle. The correct choice states that cosine is defined as x/r, where x is the length of the adjacent side, and r is the length of the hypotenuse.

When you consider a unit circle, the coordinates of a point on the circle can be represented by (x, y), where x corresponds to the cosine of the angle, and r (the hypotenuse) is always equal to 1 since it is a unit circle. Therefore, the cosine of an angle can also be interpreted as the x-coordinate of the point on the unit circle corresponding to that angle. This fundamental relationship reinforces why the correct answer is x/r.