Prove that in polar coordinates for any given point P on the curve , the angle from the radius vector CP to the tangent line at P is given by

I understand the proof given here if we assume the given lengths, but I can't make out the diagram and I don't understand why the given lengths are true; why PR = for instance. Could someone provide a better diagram or explain it? Thanks.

http://wstein.org/home/wdj/teaching/...us_vector.html