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 less than 180°, 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.

The sum of the internal angle and the external angle on the same vertex is 180°.

for example:

${\displaystyle x+35+75=180}$

${\displaystyle x+110=180}$

${\displaystyle x+110-110=180-110}$

${\displaystyle x=70}$

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