This sketch corresponds to p.840 #15 and shows the conclusions proven in part a, b and c. You can drag point [math]P[/math] to see other possible focal chords. Notice the conclusions: a) The tangent lines at [math]P[/math] and [math]Q[/math] are perpendicular b) The tangent lines meet on the directrix. In this diagram, their point of intersection is labeled [math]D[/math] c) The segment [math]\overline{FD}[/math] is perpendicular to the focal chord [math]\overline{PQ}[/math] Use the checkboxes to show/hide the various features of the graph