Rotate point G until segment i and segment h are "flush" with each other. Then the bottom triangle has a hypotenuse of 13 (radius of sphere) and the two legs of the bottom triangle are 5 (distance from center of sphere to new circle) and r (the radius of the new circle that we are trying to find). 13^2-5^2=r^2 12^2=r^2 12=r