You can say the ring is a circle, but that's not the point.

I'm trying to figure out how to increase the radius of a ring (or a circle with 1px border size) without moving the circle from the center point. Usually, I plan out the algorithm by using math equations, before implementing the code. I haven't created any code, because I'm stuck on this math problem which hindered me from advancing forward.

From a math equation, this is the best one that contains all the necessary ingredients for my code:

r = Math.sqrt((x*x) + (y*y));

Rearranging it, I get this:

y = Math.sqrt((x*x) - (r*r));

But I couldn't get my head to wrap around how to increase the radius without touching the X and Y values. How do I solve this?