Focal Definition of Ellipse

The applet below illustrates the focal property of the ellipse: given two points F and F' (called the foci of the ellipse), ellipse is the locus of points P the sum of whose distances to the foci is constant: PF + PF' = const. This property is often taken for a definition of ellipse. (A different demonstration can be found elsewhere.)


This applet requires Sun's Java VM 2 which your browser may perceive as a popup. Which it is not. If you want to see the applet work, visit Sun's website at, download and install Java VM and enjoy the applet.

What if applet does not run?

Conic Sections > Ellipse

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