Obtuse Angle

Pronunciation: /əbˈtus ˈæŋ gəl/ Explain

Obtuse Angles
Figure 1: Obtuse angles measure > 90° and <180°

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Angles are divided into four classes according to their measure. An obtuse angle is an angle that measures more than 1/4 of a full circle and less than 1/2 of a full circle[1][2].

Classes of Angles
Degree MeasureRadian MeasureClass Name
α<90°α<π/2 rad. Acute angle
α=90°α=π/2 rad. Right angle
90°<α<180°π/2<α<π rad. Obtuse angle
α=180°α=π rad. Straight angle
180<α<360°π<α<2π rad. Reflex angle
α=360°α=2π rad. Full angle
Table 1: Classes of angles

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  1. Keller, Samuel Smith. Mathematics for Engineering Students, Plane and Solid Geometry. pg 3. www.archive.org. D. Van Nostrand Company. 1908. Last Accessed 1/2/2010. http://www.archive.org/stream/mathengineer00kellrich#page/3/mode/1up/search/obtuse. Buy the book
  2. Casey, John, LL.D., F.R.S.. The First Six Books of the Elements of Euclid. pg 7. www.archive.org. Casey, John, LL.D. F.R.S.. Hodges, Figgis & Co.. 1890. Last Accessed 1/2/2010. http://www.archive.org/stream/firstsixbooksofe00caseuoft#page/7/mode/1up/search/obtuse. Buy the book
  3. obtuse angle. merriam-webster.com. Encyclopedia Britannica. Merriam-Webster. Last Accessed 12/21/2009. http://www.merriam-webster.com/dictionary/obtuse angle.

More Information

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

Cite this article as:

McAdams, David E. Obtuse Angle. 8/7/2018. All Math Words Encyclopedia. Life is a Story Problem LLC. http://www.allmathwords.org/en/o/obtuseangle.html.

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Revision History

8/7/2018: Changed vocabulary links to WORDLINK format. (McAdams, David E.)
1/1/2010: Added "References". (McAdams, David E.)
12/21/2009: Added reference to Euclid's Elements; Expanded table of angle classes. (McAdams, David E.)
9/16/2008: Changed text on classes of angles to a table. Changed images 2 and 3 to one manipulative (McAdams, David E.)
8/26/2008: Added entries to 'More Information'. (McAdams, David E.)
7/12/2007: Initial version. (McAdams, David E.)

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