(x-s)^2+(y-p)^2=p^2
s^2+(a-p)^2=p^2
s^2+(b-p)^2=p^2
(a-p)^2=(b-p)^2
p=(a+b)/2
s^2=p^2-(p-a)^2
...
(x-(ab)^.5)^2+(y-(a+b)/2)^2=((a+b)/2)^2