of a Parabola
A line perpendicular to
of symmetry used in the definition of a parabola.
A parabola is defined as follows: For a given point, called the
focus, and a given line
not through the focus, called the directrix, a parabola is the
locus of points such that the distance to the focus equals the
distance to the directrix.
an ellipse, directrices of a hyperbola