r=|m-2m- 1|/√( 1^2+ 1^2)=|m+ 1|/√2;
Let the equation of the circle be (x-m) 2+(y+2m) 2 = r 2, and substitute the coordinates of point A (2,-1) into the circle to get the solution of m = 1+√ 2 or m = 1-√ 2.
When m = 1+√ 2, r =1+√ 2;
When m = 1-√ 2, R = 1-√ 2/2.