a;dungphuong phap toa do: phai co toa do cu the cua A;B;CM(xM;yM) A(xA;yA) B(xB;yB)C(xC;yC)
ta co:
$sqrt((xA-xM+xC-xB)^2+(yA-yM+yC-yB)^2)=sqrt(((xA-xM)*2/3+xB-xM)^2+((xA-xM)*2/3+xB-xM)^2)$
tuong duong
xM^2+yM^2+2xM*TX+2yM*TY+T(A,B,C)=xM^2/9+yM^2/9+2xM*GX+2yM*GY+G(A,B)
trong do, T la cac ham chua A,B,C va G la ham chua toa do cua A,B; TX la ham chua toa do x cua A,B,C, TY la ham chua toa do y cua A,B,C. G tuong tu.
xM^2-xM^2/9+2xM*HX+yM^2-yM^2/9+2yM*HY+H(A;B;C)=0
8/9xM^2-2xM*HX+8/9yM^2-2yM*HY+H=0
xM^2-2xM*HX*9/8+yM^2-2yM*HY*9/8+9/8*H=0
tuong duong
(xM-D)^2+(yM-E)^2=I(A;B;C)
Neu I(A;B;C)<=0; khong ton tai diem M nhu vay.
Neu I(A;B;C)>0: quy tich diem M la duong tron tam (D;E) ban kinh sqrt(I(A;B;C))