Undecagon  (11-gon, Hendecagon)
Latin: una "one" + Greek: deka "ten" + gonia "angle"
Definition: A polygon with 11 sides
Try this Adjust the undecagon below by dragging the orange dot on any vertex. By clicking on the top left command line, you can switch it between a regular and irregular undecagon.

Some say that the word undecagon is incorrect because it is derived from Latin (una - "one") whereas most polygon names are Greek in origin. By that rule it would be called a hendecagon instead. To avoid confusion simply call it an 11-gon.

Properties of regular undecagons

Interior angle 147° Like any regular polygon, to find the interior angle we use the formula   (180n–360)/n . For a undecagon, n=11. See Interior Angles of a Polygon
Exterior Angle 33° To find the exterior angle of a regular undecagon, 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 9.365s2
approx
Where S is the length of a side. To find the exact area of a undecagon or any polygon, using various methods, see Area of a Regular Polygon and Area of an Irregular Polygon

Properties of all undecagons

Number of diagonals 44 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 9 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 1620° In general 180(n–2) degrees . See Interior Angles of a Polygon

Undecagonal coins

The Canadian dollar coin and the US Susan B Anthony dollar are undecagonal in shape. While the US dollar is actually round, it appears to have 11 sides because of the undecagon inscribed just inside the rim, as seen below.

Related polygon topics

General

Types of polygon

Area of various polygon types

Perimeter of various polygon types

Angles associated with polygons

Named polygons

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