Definition of a Parabola
Line AB is the directrix for the Parabola below. Point C is the focus of the Parabola below. Point D lies on the Parabola below. d1 is the distance from the focus to point D. d2 is the distance from the directrix to point D. Move points A or B to change the directrix. Move point C to change the location of the focus. Then move point D around the Parabola.
What do you notice about the distances d1 and d2 as you move point D around the Parabola? How does this applet dynamically verify our definition for a Parabola?