A decagon is a shape with 10 sides and 10 corners (vertices), and usually refers to a regular decagon, having all sides of equal length and each internal angle equal to 144°.
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.
By simple trigonometry .
The side of a regular decagon inscribed in a unit circle is , where ϕ is the golden ratio, .