

Undecagon (11gon, 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 11gon.
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
180interior angle.
See Exterior Angles of a Polygon 
Area 
9.365s^{2} 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.
Other 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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