In the diagram, below, we have two given points, [math]F_1[/math] and [math]F_2[/math]. The distances from these two points to a point [math]P[/math], given by [math]r_1[/math] and [math]r_2[/math] always add up to the same amount.[br][b]Definition of an Ellipse:[/b] The set of all points, the sum of whose distances to two fixed points is a constant.

In the diagram, below, we have two given points, [math]F_1[/math] and [math]F_2[/math]. The distances from these two points to a point [math]P[/math], given by [math]r_1[/math] and [math]r_2[/math] always add up to the same amount.[br][b]Definition of an Ellipse:[/b] The set of all points, the sum of whose distances to two fixed points is a constant.