Ellipse Locus of points

Locus of points which describe an ellipse. The ellipse is all points where the sum of the distances from two points (the foci) is a fixed constant.
The slider changes the partial distance on a line segment. The remaining distance also changes, but the sum of these two distances remains the same. These two partial line segments are the radii for two circles. Where the circles intersect, this is the locus of points for the ellipse. To see the construction of the worksheet, experiment with moving the two focii around and changing lengths of different line segments.