Decagon
In geometry, a decagon is a ten-sided polygon or 10-gon.
Regular decagon
A regular decagon has all sides of equal length and each internal angle will always be equal to 144°. Its Schläfli symbol is {10} and can also be constructed as a truncated pentagon, t{5}, a quasiregular decagon alternating two types of edges.
The area of a regular decagon is: (with t = edge length)
An alternative formula is 
where d is the distance between parallel sides, or the height when the decagon stands on one side as base, or the diameter of the decagon's inscribed circle.
By simple trigonometry,
and it can be written algebraically as
Sides
The side of a regular decagon inscribed in a unit circle is 
, where ϕ is the golden ratio, 
.
Construction
As 10 = 2 × 5, a power of two times a Fermat prime, it follows that a regular decagon is constructible using compass and straightedge, or by an edge-bisection of a regular pentagon.
An alternative (but similar) method is as follows:
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