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Regular hendecagon
Edges and vertices 11
Schläfli symbols {11}
Coxeter–Dynkin diagrams
Symmetry group Dihedral (D11)
Area
(with $a$ = edge length)
$A=\frac{11}{4}\cot\left(\frac{\pi}{11}\right)a^2$$\simeq 9.36a^2$
Internal angle
(degrees)
180°×(1-2/11)
=147°.27

In geometry, a hendecagon (also undecagon[1]) is an 11-sided polygon. The name "undecagon" is often seen as incorrect, but the matter is up for debate. The Greek prefix 'hen', is preferable to the Latin 'uni' or 'un'.[2] A regular hendecagon has internal angles of 147.27 degrees. The area of a regular hendecagon with side length $a$ is given by

$A=\frac{11}{4}\cot\left(\frac{\pi}{11}\right)a^2\simeq9.36a^2$

A regular hendecagon is not constructible with compass and straightedge.

## Use in coinage

The Canadian dollar coin, the loonie, is patterned on a regular hendecagonal prism, as is the Indian two-rupee coin.

It was also patterned on the Susan B. Anthony dollar of the United States from 1979-1981 and again in 1999.