The Foci of an Ellipse

This worksheet illustrates the relationship between an ellipse and its foci. Move the yellow point along the ellipse.
What are the red points called?
What is the relationship between each of the green (resp. blue) line segments?
What happens to the [i]sum [/i]of the lengths of the green and blue line segments as the yellow point moves along the ellipse?
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– Kristen Beck

Information: The Foci of an Ellipse