what's the eqution of directrix of a circle & what are the coordinates of its focii?

Question

circle has an eccentricity 0

Answer 1

It's improper to talk about the "foci" of a circle, because a circle has only a single focus, its center. The directrix of a circle has no equation because the directrix lies at an infinite distance from the circle. In fact, the proper name for the directrix of a circle is "the line at infinity," and this line has other applications as well.

Answer 2

The directrix and the foci of a circle are just the circle's center.