The terminal side of -pi/2 lies along?

Question

 

OX, OX’, OY or OY’

how is it OY’ and not OY?

Answer 1

While it's not entirely clear from the statement of your problem, let's suppose that by OX, OY you mean the positive x and y axes, respectively, and by OX' and OY' you mean the negative x and y axes, respectively. Then -pi/2 represents a CLOCKWISE 90 degree rotation about the origin O. (Counterclockwise rotations are represented by positive angles.) This brings the starting point (1,0) to the point (0,-1), that is, to OY'.