You draw a square. Then, you draw the largest possible circle, tangent to all four sides. If the lower left corner of the square is at (0,0), and the sides of the square are parallel to the coordinate axis, the point (2,1) is on the circle, within the square.

What is the radius of the circle?

(In reply to

Circular Argument by Kenny M)

Yes, you nailed it down! You got the exact reason why I wrote "within the square" -- and I did hope ;-) some would miss it but others wouldn't...