Focus

Pronunciation: /ˈfoʊ.kəs/ Explain

Figure 1: Focus

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. An ellipse and a hyperbola both have two focuses.

References

1. McAdams, David E.. All Math Words Dictionary, focus. 2nd Classroom edition 20150108-4799968. pg 81. Life is a Story Problem LLC. January 8, 2015. Buy the book
2. Hustler, James Devereux. The Elements of the Conic Sections with the Sections of the Conoids. 3rd edition. pp 1-18. www.archive.org. J. Deighton and Sons. 1826. Last Accessed 7/11/2018. http://www.archive.org/stream/elementsofconics00hastuoft#page/1/mode/1up/search/focus. Buy the book
3. Loomis, Elias. Elements of geometry and conic sections. www.archive.org. Harper & Brother, Publishers. 1849. Last Accessed 7/11/2018. http://www.archive.org/stream/elementsgeometr05loomgoog#page/n183/mode/1up/search/focus. Buy the book

More Information

• McAdams, David E.. Circle. allmathwords.org. All Math Words Encyclopedia. Life is a Story Problem LLC. 3/12/2009. http://www.allmathwords.org/en/c/circle.html.

Cite this article as:

McAdams, David E. Focus. 12/21/2018. All Math Words Encyclopedia. Life is a Story Problem LLC. http://www.allmathwords.org/en/f/focus.html.

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 LLC and are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
• Focus: Seahen, commons.wikimedia.org. This work is licensed under the GNU Free Documentation License, Version 1.2. https://commons.wikimedia.org/wiki/File:Excentricidad.svg.

Revision History

12/21/2018: Reviewed and corrected IPA pronunication. (McAdams, David E.)
7/9/2018: Removed broken links, updated license, implemented new markup, implemented new Geogebra protocol. (McAdams, David E.)
1/7/2010: Added "References". (McAdams, David E.)
10/11/2008: Initial version. (McAdams, David E.)