# Foci of Ellipses

## Foci: The guiding force

Recall the major vertices of an ellipse are the end points of the major axis.
The distance from the center to the horizontal vertices is a.
The vertical distance from the center to the vertical vertices is b.
The underlying "force" of an ellipse are the foci.
They are what tie the major and minor vertices together.
Play around with the ellipse to see the foci interact with the ellipse.
If you make a=4, and b=5 or vice versa pay careful attention to the foci points!

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