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 an adjacent side.

[ Calculation

The sum of the internal angles of a simple polygon can be calculated using the formula

where the variable n represents the number of sides the polygon has. Following on from this, dividing the result by n gives the angle measure of each side.

Similarly, the measures of a given polygon's exterior angles can be calculated by dividing by the number of sides of thee polygon n.

[ External links

• 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