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Directrices 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.

 

 

 

See also

Directrices of a hyperbola, directrix of a parabola

 


  this page updated 19-jul-17
Mathwords: Terms and Formulas from Algebra I to Calculus
written, illustrated, and webmastered by Bruce Simmons
Copyright © 2000 by Bruce Simmons
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NCTM Web Bytes December 2004 Web Bytes March 2005 Web Bytes