Obtuse Angle
Pronunciation: /əbˈtus ˈæŋ.gəl/ Explain
Figure 1: Obtuse angles measure > 90° and <180° 
Click on the blue point and drag it to change the figure.
Can you find all 5 angle classes represented?
 Manipulative 1  Angle Classes Created with GeoGebra. 

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^{[2][3]}.
Classes of Angles
Degree Measure  Radian Measure  Class 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 

References
 McAdams, David E.. All Math Words Dictionary, obtuse angle. 2nd Classroom edition 201501084799968. pg 128. Life is a Story Problem LLC. January 8, 2015. Buy the book
 Keller, Samuel Smith. Mathematics for Engineering Students, Plane and Solid Geometry. pg 3. www.archive.org. D. Van Nostrand Company. 1908. Last Accessed 9/5/2018. http://www.archive.org/stream/mathengineer00kellrich#page/3/mode/1up/search/obtuse. Buy the book
 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 9/5/2018. http://www.archive.org/stream/firstsixbooksofe00caseuoft#page/7/mode/1up/search/obtuse. Buy the book
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. 4/27/2019. All Math Words Encyclopedia. Life is a Story Problem LLC. http://www.allmathwords.org/en/o/obtuseangle.html.
Image Credits
Revision History
4/27/2019: Changed equations and expressions to new format. (
McAdams, David E.)
12/21/2018: Reviewed and corrected IPA pronunication. (
McAdams, David E.)
9/5/2018: Removed broken links, updated license, implemented new Geogebra protocol, implemented new markup. (
McAdams, David E.)
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.)