# PDST Locus of Ellipse

- Topic:
- Ellipse

Recall that an ellipse is a locus (set) of points in the plane for which the

**sum**of the distances from two fixed points, called foci, remains constant. In the applet below, feel free to adjust the**black siders**and the location of the**brown point (x,y)**on the ellipse before dragging the blue slider. What is the value of this**special sum**?