Originally Posted by

**wesleybrown** I've got a question on countability like this:

Let S be the set of all circles on the coordinate plane that pass through (1,1) and another point (x*sqrt(2),x*sqrt(2)) for some x is a rational number. Determine if S is a countable set or not.

I saw someone do it by fixing x=0, using perpendicular bisector of the line joining (0,0) to (1,1) as the centers of circles. Then, S will contain uncountably many circles passing through (0,0) and (1,1). But he specifically exclude the point (0.5,0.5) as a center of one of those circles, why is that?

Note: My main focus is why the point (0.5,0.5) has to be excluded as a center of those circles?