The focus-directrix construction of an ellipse
- Author:
- Susan Addington
One definition of an ellipse: the set of points equidistant from a point (focus) and a circle (directrix).
The distance to a circle is measured perpendicular to the circle (to its tangent), so distance is measured along a radius.
Another definition of an ellipse: Given two points (the foci), the set of points P for which the sum of the distances from P to the foci is constant.
Match the diagrams to prove that these definitions are equivalent.