What is the value of cos 90°?

The value of cos 90° is 0 because, in the unit circle, the cosine of an angle represents the x-coordinate of the point where the terminal side of the angle intersects the unit circle. At 90°, the terminal side is vertical, pointing straight upwards, which corresponds to the coordinates (0, 1) on the unit circle. Here, the x-coordinate, which indicates the cosine value, is 0.

This foundational understanding of the unit circle and how angles correspond to specific coordinates is essential when working with trigonometric functions. When moving to different angles, the x-coordinates will change according to the angle's position on the circle, leading to varying values for cosine.