Ellipse by Foci - Cinderella Documentation
Description: The user's guide to the Interactive Geometry Software Cinderella

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# Ellipse by Foci

This mode constructs an ellipse defined by two foci and one perimeter point. After selecting the mode one has to click focus1, focus2, perimeter point in this order.

#### Synopsis

Ellipse by foci mode constructs an ellipse from two foci and a perimeter point.

#### Caution

Due to the continuity behavior of Cinderella it may happen that while free elements are being moved, an ellipse transforms continuously into a hyperbola. This may happen if one of the foci passes through infinity.

#### See Also

- Hyperbola by Foci
- Parabola by Focus and Directrix