directrix

Examples of directrix in the following topics:

• Eccentricity

  • Recall that hyperbolas and non-circular ellipses have two foci and two associated directrices, while parabolas have one focus and one directrix.
  • In the next figure, each type of conic section is graphed with a focus and directrix.
  • The orange lines denote the distance between the focus and points on the conic section, as well as the distance between the same points and the directrix.
  • In other words, the distance between a point on a conic section and its focus is less than the distance between that point and the nearest directrix.
  • This indicates that the distance between a point on a conic section the nearest directrix is less than the distance between that point and the focus.

• Conics in Polar Coordinates

  • Previously, we learned how a parabola is defined by the focus (a fixed point) and the directrix (a fixed line).
  • We can define any conic in the polar coordinate system in terms of a fixed point, the focus $P(r,θ)$ at the pole, and a line, the directrix, which is perpendicular to the polar axis.
  • With this definition, we may now define a conic in terms of the directrix: $x=±p$, the eccentricity $e$, and the angle $\theta$.
  • For a conic with a focus at the origin, if the directrix is $x=±p$, where $p$ is a positive real number, and the eccentricity is a positive real number $e$, the conic has a polar equation:
  • Any conic may be determined by three characteristics: a single focus, a fixed line called the directrix, and the ratio of the distances of each to a point on the graph.  

• What Are Conic Sections?

  • For example, each type has at least one focus and directrix.
  • A directrix is a line used to construct and define a conic section.
  • As with the focus, a parabola has one directrix, while ellipses and hyperbolas have two.
  • The point halfway between the focus and the directrix is called the vertex of the parabola.
  • The vertex lies at the midpoint between the directrix and the focus.

• Conic Sections in Polar Coordinates

  • One of the most useful definitions, in that it involves only the plane, is that a conic consists of those points whose distances to some point—called a focus—and some line—called a directrix—are in a fixed ratio, called the eccentricity.
  • In the focus-directrix definition of a conic, the circle is a limiting case with eccentricity 0.

• Conic Sections

  • There are a number of other geometric definitions possible, one of the most useful being that a conic consists of those points whose distances to some other point (called a focus) and some other line (called a directrix) are in a fixed ratio, called the eccentricity.
  • In the focus-directrix definition of a conic, the circle is a limiting case with eccentricity 0.

• Parabolas As Conic Sections

  • All parabolas have a directrix.
  • The directrix is a straight line on the opposite side of the parabolic curve from the focus.
  • The parabolic curve itself is the set of all points that are equidistant (equal distances) from both the directrix line and the focus.

• Standard Equations of Hyperbolas

  • The eccentricity e equals the ratio of the distances from a point P on the hyperbola to one focus and its corresponding directrix line (shown in green).

• Parts of an Ellipse

  • It also shows how the sum of the distances from any point on the ellipse to the two foci is a constant, and how the eccentricity is determined by relating one of the foci to a line $D$ called the directrix.

• Introduction to Hyperbolas

  • The set of all points such that the ratio of the distance to a single focal point divided by the distance to a line (the directrix) is greater than one

• Nonlinear Systems of Equations and Problem-Solving

  • In the focus-directrix definition of a conic, the circle is a limiting case of the ellipse with an eccentricity of $0$.