就是圆经过A(a,0),B(b,0),C(0,c)
则圆心在AB垂直平分线上
圆心横坐标(a+b)/2
设圆心纵坐标是k
则AO=CO
[(a+b)/2-a]^2+k^2=[(a+b)/2]^2+(k-c)^2
[(b-a)/2]^2+k^2=[(a+b)/2]^2+k^2-2kc+c^2
2kc=ab+c^2
k=ab/2c+c/2
则半径平方=[(a+b)/2-a]^2+k^2
=[(b-a)/2]^2+(ab/2c+c/2)^2
所以[x-(a+b)/2]^2+[y-(ab/2c+c/2)]^2=[(b-a)/2]^2+(ab/2c+c/2)^2