Directrix (Conic Section)

$\begin{figure}\begin{center}\BoxedEPSF{Directrix.epsf scaled 750}\end{center}\end{figure}$

The Line which, together with the point known as the Focus, serves to define a Conic Section.

References

Coxeter, H. S. M. Introduction to Geometry, 2nd ed. New York: Wiley, pp. 115-116, 1969.

Coxeter, H. S. M. and Greitzer, S. L. Geometry Revisited. Washington, DC: Math. Assoc. Amer., pp. 141-144, 1967.