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)=04+a(18)=0a=-2/9x^2+y^2-x-y-2-2/9*(x^2+y^2+4x-4y)=07x^2+7y^2-17x-y-18=0
