The CabriJava applet shows a conic with given focus and directrix and eccentricity e
You can move the focus, and can change the directrix by dragging the red points. You
can vary the eccentricity by dragging the point e.

The program will calculate e and show the equation of the conic. It ia always quadratic
in x and y. Sometimes there is no xy term - when?

If e remains fixed, the theory says that the shape will remain fixed, but the figure may
change size. The cabri does not really convince me on this.

Part of the figure may be out of the picture. Indeed, when e > 1, a complete
branch may be missing.

As we have seen, the shape of the conic depends on the value of e, and there are three
basic types, depending on whether e < 1, e = 1, or e > 1. These are called, respectively,
ellipses, parabolas and hyperbolas.

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