You might want to check your lecture notes or text on this one, because I'm guessing, but...
How would you be able to calculate the radius at any given point along an ellipse? I mean, a formula for the ellipse would have to have a varying radius built into it.
This formula works for circles, but I would say not for ellipses, maybe that formula has factors for the focus instead of radius?
If you post your question in the engineering and physics sections also (assuming you haven't yet), you may get a for sure.
Good luck.
