Focus

Pronunciation: /ˈfoʊ kəs/ ?

Figure 1: Focus Image courtesy Seahen.

In geometry, a focus is a point related to the construction and properties of conic sections. A circle and a parabola have one focus. The focus of a circle is also center of the circle. Sometimes, it is convenient to talk of a circle having two coincidental focuses, meaning two focuses that occupy the same point. Similarly, a parabola is sometimes considered to have a second focus at infinity. A ellipse and a hyperbola both have two focuses.

References

1. focus. http://wordnet.princeton.edu/. WordNet. Princeton University. (Accessed: 2011-01-08). http://wordnetweb.princeton.edu/perl/webwn?s=focus&sub=Search+WordNet&o2=&o0=1&o7=&o5=&o1=1&o6=&o4=&o3=&h=.
2. Author unknown. The Elements of the Conic Sections with the Sections of the Conoids, 3rd ed., pp 1-18. Published by J. Deighton and Sons, 1826. (Accessed: 2010-01-07). http://www.archive.org/stream/elementsofconics00hastuoft#page/1/mode/1up/search/focus.
3. Loomis, Elias. Elements of geometry and conic sections. Harper & Brother, Publishers, 1849. (Accessed: 2010-02-04). http://www.archive.org/stream/elementsgeometr05loomgoog#page/n183/mode/1up/search/focus.

More Information

• McAdams, David. Circle. allmathwords.org. All Math Words Encyclopedia. Life is a Story Problem.org. 2009-03-12. http://www.allmathwords.org/article.aspx?lang=en&id=Circle.
• McAdams, David. Hyperbola. allmathwords.org. All Math Words Encyclopedia. Life is a Story Problem.org. 2009-03-12. http://www.allmathwords.org/article.aspx?lang=en&id=Hyperbola.
• McAdams, David. Parabola. allmathwords.org. All Math Words Encyclopedia. Life is a Story Problem.org. 2009-03-12. http://www.allmathwords.org/article.aspx?lang=en&id=Parabola.

Printed Resources

Cite this article as:

Focus. 2010-01-07. All Math Words Encyclopedia. Life is a Story Problem.org. http://www.allmathwords.org/en/f/focus.html.

Translations

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Image Credits

• All images and manipulatives are by David McAdams unless otherwise stated. All images by David McAdams are Copyright © Life is a Story Problem.org and are licensed under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 License.

Revision History

2010-01-07: Added "References" (McAdams, David.)
2008-10-11: Initial version (McAdams, David.)