Sorry, don't have a visual at the moment, but there's a circle with the center at: (1/2)a , with a radius of: (1/2)a

It's not centered at the origin, so I'm confused as to what theta values the circle will be going from. I think it goes from 0 to 2pi, can anybody explain what theta is supposed to be? And would I be correct in assuming that r goes from 0 to a?