a. The general equation for a circle centered at
with radius
is

The described circle has equation

We know the circle passes through the origin. This means that the equation above holds for
and
. The distance between any point on the circle and its center is the radius, so we can use this fact to determine
:

So the circle's equation is

b. If the distance between point B and the center is less than
, then B lies inside the circle. If the distance is greater than
, it falls outside the circle. Otherwise, if the distance is exactly
, then B lies on the circle.
The distance from B to the center is

, so
, which means B falls outside the circle.