A directrix is a fixed line used in the description of a curve or surface, such as a parabola, ellipse, or hyperbola. It helps define the locus of points that form these conic sections.
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For a parabola, the distance from any point on the parabola to the directrix is equal to its distance to the focus.
Ellipses and hyperbolas have two directrices, one corresponding to each focus.
In polar coordinates, equations involving conic sections can be derived using the directrix and eccentricity.
The general equation for a conic section can incorporate the directrix by using its distance formula.
Directrices are perpendicular to the major axis in an ellipse and hyperbola.