# Exterior Angle

Pronunciation: /ɪkˈstɪər.i.ər ˈæŋ.gəl/ Explain
1.  Figure 1: Exterior angle

The exterior angle of a polygon is made by extending one of the sides at a vertex, then taking the angle between the extended side and side of the polygon.[3][4] Note that the sum of the measure of an interior angle of a polygon and the measure of the corresponding exterior angle is 180° or π radians.

2.  Figure 2: Interior angle between parallel lines.

When two parallel lines are intersected by a third line (called a transversal), the angles on the outside of the parallel lines are the exterior angles. Note that the sum of the measure of the exterior angle and the measure of the corresponding interior angle is 180° or π radians.

### References

1. McAdams, David E.. All Math Words Dictionary, exterior angle. 2nd Classroom edition 20150108-4799968. pg 76. Life is a Story Problem LLC. January 8, 2015. Buy the book
2. exterior angle. merriam-webster.com. Encyclopedia Britannica. Merriam-Webster. Last Accessed 7/9/2018. http://www.merriam-webster.com/dictionary/exterior angle. Buy the book
3. Casey, John, LL.D., F.R.S.. The First Six Books of the Elements of Euclid. pp 8,28-29. Translated by Casey, John, LL.D. F.R.S.. www.archive.org. Hodges, Figgis & Co.. 1890. Last Accessed 7/9/2018. http://www.archive.org/stream/firstsixbooksofe00caseuoft#page/8/mode/1up/search/exterior. Buy the book
4. Stöcker, K.H.. The Elements of Constructive Geometry, Inductively Presented. pg 18. Translated by Noetling, William A.M, C.E.. www.archive.org. Silver, Burdett & Company. 1897. Last Accessed 7/9/2018. http://www.archive.org/stream/elementsofconstr00noetrich#page/18/mode/1up. Buy the book

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

McAdams, David E. Exterior Angle. 4/20/2019. All Math Words Encyclopedia. Life is a Story Problem LLC. https://www.allmathwords.org/en/e/exteriorangle.html.

### Revision History

4/20/2019: Updated expressions and equations to match new format. (McAdams, David E.)
12/21/2018: Reviewed and corrected IPA pronunication. (McAdams, David E.)