Geometry For Dummies, 3rd Edition
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Everything you need to know about a polygon doesn’t necessarily fall within its sides. You may need to find exterior angles as well as interior angles when working with polygons:


  • Interior angle: An interior angle of a polygon is an angle inside the polygon at one of its vertices. Angle Q is an interior angle of quadrilateral QUAD.

  • Exterior angle: An exterior angle of a polygon is an angle outside the polygon formed by one of its sides and the extension of an adjacent side.

Interior and exterior angle formulas:
  • The sum of the measures of the interior angles of a polygon with n sides is (n – 2)180.

  • The measure of each interior angle of an equiangular n-gon is

  • If you count one exterior angle at each vertex, the sum of the measures of the exterior angles of a polygon is always 360°.

Check here for more practice.

About This Article

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About the book author:

Mark Ryan is the founder and owner of The Math Center in the Chicago area, where he provides tutoring in all math subjects as well as test preparation. Mark is the author of Calculus For Dummies, Calculus Workbook For Dummies, and Geometry Workbook For Dummies.

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