How many focus and directrix does a circle have?
William Rodriguez
Published Jan 11, 2026
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.
Do circles have a directrix?
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 many focus point does a circle have?
Defining conics in terms of two fociA circle is the special case of an ellipse in which the two foci coincide with each other. Thus, a circle can be more simply defined as the locus of points each of which is a fixed distance from a single given focus.