设圆心坐标是O(m,-2m)
圆心O到点P的距离应等于圆心到直线x-y-1=0的距离
(m-2)²+(-2m+1)²=(m+2m-1)²/(1²+(-1)²)
m²-4m+4+4m²-4m+1=(9m²-6m+1)/2
5m²-8m+5=(9m²-6m+1)/2
m²-10m+9=0
(m-1)(m-9)=0
解得m=1或m=9
圆心坐标是(1,-2)或(9,-18)
圆的半径是根号2或13根号2
圆的方程是(x-1)²+(y+2)²=2或(x-9)²+(y+18)²=338