You may know that Cos 0 = 1 and cos pi = -1
If you keep adding 2 pi to the angle, the value doesn't change
So, cos pi + 2 pi = cos 3 * pi is also = - 1. Similarly, cos 5 * pi, cos 7 * pi, cos 9 * pi etc. in general cos (2 * n + 1) * pi, where n is an integer like 1, 2, 3, etc, are -1