The cosine (cos) of 90 degrees is zero. This value is taken from the unit circle, a commonly used device in mathematics that assigns values to the trigonometric functions of sine and cosine.
The cosine values around the unit circle range from 1 to -1. Cosine has a value 1 at zero degrees and a value of -1 at 180 degrees. The other values assigned to cosine by the unit circle are 0.87 at 30 degrees, 0.71 at 45 degrees and 0.50 at 60 degrees. These values alternate between positive and negative depending on the quadrant in which they lie.