You can evaluate cos 180 using the unit circle.
The unit circle is a circle with a radius of 1, centered at the origin of a coordinate plane. Each point on the unit circle corresponds to an angle in standard position, and the coordinates of that point represent the cosine and sine of that angle.
When the angle is 180 degrees, the point on the unit circle is (-1, 0). The x-coordinate of this point represents the cosine of 180 degrees.
Therefore, cos 180 = -1.
