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 6.182s2
approx
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

Related polygon topics

General

Types of polygon

Area of various polygon types

Perimeter of various polygon types

Angles associated with polygons

Named polygons