of an Ellipse
Two parallel lines on the outside
of an ellipse perpendicular to the major
axis. Directrices can
be used to define an ellipse. Formally, an ellipse is the locus of points such that the ratio of the distance to the nearer focus to the distance
to the nearer directrix equals a constant that
is less than one. This constant is the eccentricity.
of a hyperbola, directrix of a parabola