2a+b=0

2a+b=0
b=-2a
(2-a)^2+(-1-b)^2=r^2
(2-a)^2+(-1+2a)^2=r^2
5a^2-8a+5=r^2=(a-b-1)^2/2=(3a-1)^2/2
10a^2-16a+10=(3a-1)^2=9a^2-6a+1
a^2-10a+9=0
a=1 a=9
a=1,b=-2,
|a-b-1|=2 r=2/√2=√2
a=9,b=-18,
|a-b-1|=26 r=26/√2=13√2