The ellipse
An ellipse is a set of points in a plane whose distances from two fixed points (called foci) have a constant sum. As you slide the slider, this definition is illustrated. The value of this constant sum is the length of the ellipse's major axis. You can drag point P anywhere on this ellipse to see the illustration.
The equation of an ellipse centered at the origin with foci and is given in this interactive figure. Its semimajor axis, and its semiminor axis and distance, c, from the ellipse's center to any one of its foci are also displayed here. You can adjust the graph of the ellipse by moving the larger black points.
Developed for use with Thomas' Calculus, published by Pearson.