[Math] Equation for concentric circles

Question

I want an equation for concentric circles. In following image I am trying to draw concentric circles in Java but as you can see these are messed up.

This is because their (x,y) coordinates (i.e. positions) are measured from the upper-left corner, but because of having different heights and widths both circles no longer remain concentric. So I need to know how much I should increment x and y for smaller circle. Can anyone provide some equation?

Following are the methods I am using to draw circles in Java:

    g.setColor( Color.YELLOW );
    g.fillOval( 10, 10, 300, 300 ); // x,y, width,height

    g.setColor( Color.CYAN );
    g.fillOval( 10, 10 , 200, 200 ); // x,y, width,height

Answer 1

As you can see in your picture, your two circles have different origins, but concentric circles must share the same origin. Specifically, the first circle has an origin of $(160,160)$ ($10 + 150$ radius) while the second circle has an origin of $(110,110)$ ($10 + 100$ radius). To equalize the two origins, simply add 50 to the $x$ and $y$ upper-left corner for the second circle:

g.setColor(Color.YELLOW);
g.fillOval(10, 10, 200, 200);

g.setColor(Color.CYAN);
g.fillOval(60, 60, 100, 100);

Here is another example with a generalized equation:

int origin = 160;
int diameter1 = 300;
int diameter2 = 200;
int diameter3 = 100;

g.setColor(Color.YELLOW);
g.fillOval((origin - (diameter1 / 2),
           (origin - (diameter1 / 2), diameter1, diameter1);

g.setColor(Color.CYAN);
g.fillOval((origin - (diameter2 / 2),
           (origin - (diameter2 / 2), diameter2, diameter2);

g.setColor(Color.MAGENTA);
g.fillOval((origin - (diameter3 / 2),
           (origin - (diameter3 / 2), diameter3, diameter3);