# 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
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

1. McAdams, David E.. All Math Words Dictionary, obtuse angle. 2nd Classroom edition 20150108-4799968. pg 128. Life is a Story Problem LLC. January 8, 2015. Buy the book
2. 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
3. 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

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

McAdams, David E. Obtuse Angle. 4/27/2019. All Math Words Encyclopedia. Life is a Story Problem LLC. https://www.allmathwords.org/en/o/obtuseangle.html.

### 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.)