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.

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 | 9.365s^{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 |

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 |

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.

- Polygon general definition
- Quadrilateral
- Regular polygon
- Irregular polygon
- Convex polygons
- Concave polygons
- Polygon diagonals
- Polygon triangles
- Apothem of a regular polygon
- Polygon center
- Radius of a regular polygon
- Incircle of a regular polygon
- Incenter of a regular polygon
- Circumcircle of a polygon
- Parallelogram inscribed in a quadrilateral

- Square
- Diagonals of a square
- Rectangle
- Diagonals of a rectangle
- Golden rectangle
- Parallelogram
- Rhombus
- Trapezoid
- Trapezoid median
- Kite
- Inscribed (cyclic) quadrilateral

- Regular polygon area
- Irregular polygon area
- Rhombus area
- Kite area
- Rectangle area
- Area of a square
- Trapezoid area
- Parallelogram area

- Perimeter of a polygon (regular and irregular)
- Perimeter of a triangle
- Perimeter of a rectangle
- Perimeter of a square
- Perimeter of a parallelogram
- Perimeter of a rhombus
- Perimeter of a trapezoid
- Perimeter of a kite

- Exterior angles of a polygon
- Interior angles of a polygon
- Relationship of interior/exterior angles
- Polygon central angle

- Tetragon, 4 sides
- Pentagon, 5 sides
- Hexagon, 6 sides
- Heptagon, 7 sides
- Octagon, 8 sides
- Nonagon Enneagon, 9 sides
- Decagon, 10 sides
- Undecagon, 11 sides
- Dodecagon, 12 sides

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