"A hypocycloid is a curve traced by a fixed point P on a circle C with radius b as C rolls on the inside of a circle with the center on the origin and radius a. If the initial point P is (a,0) and the parameter is theta, then the parametric equation is

x = (a-b)cos(theta) + bcos((theta)(a-b)/b))

y = (a-b)sin(theta) + bsin((theta)(a-b)/b))

Question 1:Show that the parametric equation is correct"

This is the picture I drew out to clarify..

But I don't even know how to start explaining why it is correct... any suggestions?