# On geometric definition of ellipse

The applet demonstrates the following: An ellipse is the set of all points in the plane, the sum of whose distances to two fixed points (foci) remains constant.
• Select the length of a piece of string by dragging the endpoints of the blue segment.
• Drag the orange point to select the position of the focus F1 along the x-Axis or the y-Axis. The other focus F2 is symmetrical to F1 with respect to the origin.
A string with the selected length is attached to both foci and is kept tight by the tip of the pencil.
• Drag the tip of the pencil or press the “Draw” button to trace all points on the plane that satisfy the above definition.
• 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.
• Bring the two foci to the origin to see the circle as a special case of the ellipse.
• Click on “Labels” to see some terminology.