What (not-too-well-known) theorem about a parabola is dynamically being illustrated in the applet below? [br](Feel free to move any of the moveable points anywhere you'd like!)
[b]Theorem:[/b][br][color=#9900ff][b][br]Suppose [i]P[/i] is a point that lies on a parabola.[/b][/color] Suppose line [i]a[/i] is the line that passes through the parabola's vertex and is perpendicular to the parabola's axis of symmetry (or is parallel to the parabola's directrix.) Let [i]V[/i] = the vertex of the parabola and [i]R[/i] = the point at which the line through [i]P[/i] that's perpendicular to the directrix meets line [i]a[/i]. If this is the case, then the line tangent to the parabola at [i]P[/i] WILL ALWAYS bisect segment [i]VR[/i].