== '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.