Hi, I am trying to follow the logic of tis problem and the solution.

cos (- 7pi/2) = cos(-4pi + pi/2) = cos pi/2 = 0.

Why do we use -4pi + pi/2 to get pi/2? How do we know to add -4pi + pi/2?

Please help me to understand what's going on with this one.

Thanks a lot for any help.