Why do I need a coterminal angle for cos(-pi/2) for the unit circle?

According to sign rules, any negative angle given to cos is equal to the same thing with a positive sign. So essentially:

cos(-pi/2) = cos(pi/2)

In my textbook, it tells me to calculate a coterminal angle of 3pi/2 and use the unit circle. I'm not sure why I have to if pi/2 is already on the unit circle. The cos of 3pi/2 happens to be 0 also, but I have noticed that its sin value is -1, which is different than pi/2's. I want to know why the book is asking me to calculate a coterminal angle when I should be able to use pi/2? Or am I wrong?