Nonagon  (9-gon) (also Enneagon)
From Latin: nonus - "ninth"
Definition: A polygon with 9 sides
Try this Adjust the nonagon below by dragging any orange dot. By clicking on the top left command line, you can switch it between a regular and irregular nonagon.

## Properties of regular nonagons

 Interior angle 140° Like any regular polygon, to find the interior angle we use the formula   (180n–360)/n . For a nonagon, n=9. See Interior Angles of a Polygon Exterior Angle 40° To find the exterior angle of a regular decagon, we use the fact that the exterior angle forms a linear pair with the interior angle, so in general it is given by the formula 180-interior angle. See Exterior Angles of a Polygon Area 6.182s2approx Where S is the length of a side. To find the exact area of a decagon or any polygon, using various methods, see Area of a Regular Polygon and Area of an Irregular Polygon

## Properties of all nonagons

 Number of diagonals 27 The number of distinct diagonals possible from all vertices. (In general  ½n(n–3)  ). In the figure above, click on "show diagonals" to see them. See Diagonals of a Polygon Number of triangles 7 The number of triangles created by drawing the diagonals from a given vertex. (In general n–2). In the figure above, click on "show triangles" to see them. See Triangles of a Polygon Sum of interior angles 1260° In general 180(n–2) degrees . See Interior Angles of a Polygon
While you are here..

... I have a small favor to ask. Over the years we have used advertising to support the site so it can remain free for everyone. However, advertising revenue is falling and I have always hated the ads. So, would you go to Patreon and become a patron of the site? When we reach the goal I will remove all advertising from the site.

It only takes a minute and any amount would be greatly appreciated. Thank you for considering it!   – John Page

Become a patron of the site at   patreon.com/mathopenref