圆系方程
x^2+y^2-x-y-2+a(x^2+y^2+4x-4y)=0
代入点(3,1)
9+1-3-1-2+a(9+1+12-4)=0
4+a(18)=0
a=-2/9
x^2+y^2-x-y-2-2/9*(x^2+y^2+4x-4y)=0
7x^2+7y^2-17x-y-18=0