# Search Results for “foci”

##### Trammel foci

Altering |AB| and |AC| changes the ellipse. Can you find a relationship between |AB|, AC| and |DE|, the distance between the foci?##### Circle Envelope

The ellipse has foci A and B. The family of circles whose center C lies on an ellipse and which pass through a fixed point D defines an envelope curve. (Press**Show**). What does the envelope look like when D lies on one of the foci of the ellipse.