x^2+y^2-4x-8y-80=0
(x-2)^2+(y-4)^2=100
弦MN的中点A(3,5)
xM+xN=2xA=6,yM+yN=2yA=10
k(MN)=(yM-yN)/(xM-xN)=(y-5)/(x-3)
[(xM-2)^2+(yM-4)^2]-[(xM-2)^2+(yN-4)^2]=100-100
(xM+xN-4)*(xM-xN)+(yM+yN-8)*(yM-yN)=0
(6-4)+(10-8)*(yM-yN)/(xM-xN)=0
1+(y-5)/(x-3)=0
x+y-8=0