Directrix Concept Inapplicable to the Circle The previous version of the article used to read prior to my edit Conic sections are exactly those curves that for a point F a line L not containing F and a non negative number e are the locus of points whose distance to F equals e times their distance to L but this definition does not apply to the circle even though the circle is a bona fide conic section Attempts to save it by talking about a directrix at infinity don t work because multiplying infinity by zero gives an indeterminate not a constant value I ve modified that section in question but would welcome further refinement by an experienced mathematician explaining simply but rigorously what if anything can be saved from this definition of a conic section as applied to the circle