Students will learn what the foci of an ellipse are, in terms of distance between each focus and the ellipse. Then students will learn how to derive the focal length formula for an ellipse. Here's a lesson, and here's practice. Finally, students will learn how to use the standard form and focal length formulas to solve word problems. Here's an excellent video.

TODO: Is there a video demonstration of the whispering chamber in The Statuary Hall of the Capitol Building in Washington, DC?