A circle is a limiting case and is not defined by a focus and directrix in the Euclidean plane. The eccentricity of a circle is defined to be zero and its focus is the center of the circle, but its directrix can only be taken as the line at infinity in the projective plane.

How do you find the directrix of a circle?

The directrix formula is x = -p Since p = 2, then x = - (2) = -2 The directrix is x = -2.

Does a circle have a focus and directrix?

A circle is determined by one focus. A circle is the set of all points in a plane at a given distance from the focus (center). A parabola is determined by a focus and a directrix (a line). A parabola is the set of points in a plane such that the distance from the focus equals the distance to the directrix.