Answer: When theta equals pi, y = cos theta takes on the value of -1. This is because the cosine function, which is a mathematical function that calculates the ratio between the adjacent side of a right triangle and the hypotenuse, reaches its minimum value of -1 when its input variable theta equals pi, which is equivalent to 180 degrees in degrees. Therefore, if you substitute pi as theta into the formula y = cos theta, you get the result of y = cos pi = -1. It is worth noting that this value of -1 represents the y-coordinate on the unit circle, which is a circle of radius 1 centered at the origin (0,0) in the Cartesian plane.