Click and drag the point [math]D[/math] on [math]y=-c'[/math] to trace the locus of points that are equidistant from [math]y=-c[/math] and point [math]F[/math]. Adjust the slider for [math]c'[/math] to change the distance from the origin to the focus, [math]F[/math], and directrix, [math]y=-c'[/math].