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Internal angle
From Wikipedia, the free encyclopedia
In geometry, an interior angle (or internal angle) is an angle formed by two sides of a simple polygon that share an endpoint, namely, the angle on the inner side of the polygon. A simple polygon has exactly one internal angle per vertex.
If every internal angle of a polygon is at most 180 degrees, the polygon is called convex.
In contrast, an exterior angle (or external angle) is an angle formed by one side of a simple polygon and a line extended from that side.
To find the total measure of degrees in a regular polygon, (regular meaning all sides and angles are equal) you must take the number of sides the polygon has, n, subtract 2 from it, then multiply that number by 180.
Example:
A decagon, a polygon with 10 sides, is a simple shape to figure the total measure of
= measure in degrees, when n = number of sides
Solution to the decagon:
The total measure of the decagon is 1440º.
Divide that number by the number of sides, in this case, 10, to find the measure of each angle.
Each interior angle of a regular decagon is 144º.
The sum of all the exterior angles on a polygon is 2·360º=720° (there are two congruent angles at each vertex).
To find the measure of a regular decagon's exterior angles, divide 360° by the number of sides the polygon has, in this case, 10.
So all the exterior angles in a regular decagon are 36º.
- Internal angles of a triangle and External angles of a triangle With interactive animation
- Angle definition pages with interactive applets that are also useful in a classroom setting. Math Open Reference
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