# Constructing an Ellipse

## Goal

Find the locus that will create an ellipse.
An ellipse is the set of points such that the sum of the distances from two points is constant.

## Directions

1) Create a circle with center at A with defined radius d.
2) Create another circle centered at B and with radius a-d.
3) Move the slider until the circles intersect at two points. Label those two points C and D.
4) Create segments AC, AD, BC and BD.
5) Unselect the circles so that they do not show up.
6) Right click on C and D and turn on the trace.
7) Move the slider around to see the points C and D "create" an ellipse.

## Observations?

Given the definition of an ellipse (see above) why did we construct the circles in this way? Hint: How did we define our radii?