The applet demonstrates the following:[br][i]An ellipse is the set of all points in the plane, the sum of whose distances to two fixed points (foci) remains constant.[/i][br][list][br][*]Select the length of a piece of string by dragging the endpoints of the blue segment.[br][*]Drag the [b]orange[/b] point to select the position of the focus F1 along [b]the x-Axis or the y-Axis[/b]. The other focus F2 is symmetrical to F1 with respect to the origin.[br][/list][br]A string with the selected length is attached to both foci and is kept tight by the tip of the pencil.[br][list][br][*]Drag the tip of the pencil or press the “Draw” button to trace all points on the plane that satisfy the above definition.[br][*]Hide the pencil by pressing the "Pencil ON/OFF" button; show the ellipse by pressing “Show Ellipse” button, and explore the curve by changing the positions of the foci and the length of the constructing string. [br][*]Bring the two foci to the origin to see the circle as a special case of the ellipse.[br][*]Click on “Labels” to see some terminology.[br][/list]