(x2-2x+1)+(y2+4my+4m2)=-3m2+1+4m2
(x-1)2+(y+2m)2=m2+1
圆心(1,-2m)在x+y+2=0
1-2m+2=0
m=3/2
则r2=m2+1=13/4
所以r=√13/2