*Macmillan Science Library: Mathematics*. Copyright © 2001-2006 by Macmillan Reference USA, an imprint of the Gale Group. All rights reserved.

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## Foci

For every point on an ellipse, the *sum* of the distances from that point to two points called the foci is the same. These two points, the foci, are on the major (longest) axis of the ellipse. The value of this constant sum is 2*a*.

For every point on a hyperbola, the absolute value of the *difference* of the distances from that point to two points called the foci is the same. The absolute value of this constant difference is 2*a*.

## See Also

## Bibliography

Jacobs, Harold R. *Mathematics, A Human Endeavor.* San Francisco: W.H. Freeman and Company, 1970.

This section contains 525 words(approx. 2 pages at 300 words per page) |