Focus and Directrix of Ellipse

In the presence of a point F and a straight line d, ellipse can be characterized is the locus of points P whose distances to F and d are in a fixed ratio less than 1: dist(P, F) / dist(P, d) = const < 1. The point is called a focus and the line a directrix of the ellipse. This property is often taken for a definition of ellipse. For every ellipse there are two focus/directrix combinations. The line joining the foci is the axis of summetry of the ellipse and is perpendicular to both foci.

Conic Sections > Ellipse

• What Is Ellipse?
  
• Analog device simulation for drawing ellipses
• Angle Bisectors in Ellipse
• Angle Bisectors in Ellipse II
• Between Major and Minor Circles
• Brianchon in Ellipse
• Butterflies in Ellipse
• Conjugate Diameters in Ellipse
• Dynamic construction of ellipse and other curves
• Ellipse in Arbelos
• Ellipse Between Two Circles
• Euclidean Construction of Center of Ellipse
• Euclidean Construction of Tangent to Ellipse
• Focal Definition of Ellipse
• Focus and Directrix of Ellipse
• From Foci to a Tangent in Ellipse
• Gergonne in Ellipse
• Pascal in Ellipse
• La Hire's Theorem in Ellipse
• Optical Property of Ellipse
• Parallel Chords in Ellipse
• Poncelet Porism in Ellipses
• Reflections in Ellipse
• Three Squares and Two Ellipses
• Three Tangents, Three Chords in Ellipse
• Van Schooten's Locus Problem