Decagon: A Perfect 10-Sided Polygon

In the world of geometry, the decagon shines brightly as a fascinating shape. With its ten sides and ten angles, this polygon is truly a unique gem. In this article, we will explore the properties...

In the world of geometry, the decagon shines brightly as a fascinating shape. With its ten sides and ten angles, this polygon is truly a unique gem. In this article, we will explore the properties and characteristics of the decagon, unraveling its secrets and beauty. So, let's dive in!

Unveiling the Regular Decagon

A regular decagon is a special type of decagon with sides of equal length and internal angles measuring 144°. Its Schläfli symbol is {10}, and it can also be constructed as a truncated pentagon, t{5}, which alternates two types of edges. The regular decagon exudes symmetry and elegance.

Side Length: An Intriguing Connection

To better understand the regular decagon, let's take a look at its side length. In the image below, you can see a regular decagon with side length a and radius R of the circumscribed circle.

Regular Decagon

The triangle E10E1M has two equally long legs with a length of R and a base with a length of a. The circle around E1 with radius a intersects [M, E10] in a point P. Now, the triangle E10E1P is an isosceles triangle with vertex E1 and base angles measuring 72°.

Therefore, ME1P is also an isosceles triangle with vertex P. The length of its legs is a, so the length of [PE10] is R - a. By utilizing these measurements, we can derive the equation: a^2 = R^2 - aR. This equation enables us to find the side length, a, of the regular decagon.

Exploring the Area of a Decagon

The area of a regular decagon can be calculated in various ways, depending on the given parameters. In terms of side length a, the area is given by:

A = (5/2) a^2 cot(π/10) ≈ 7.694208843 a^2*

Alternatively, in terms of the apothem r, the area is:

A = 10 tan(π/10) r^2 ≈ 3.249196962 r^2*

Lastly, in terms of the circumradius R, the area is:

A = 5 sin(π/5) R^2 ≈ 2.938926261 R^2*

The area of a regular decagon can also be represented by the formula A = 2.5 da, where d is the distance between parallel sides or the height when the decagon stands on one side as the base. By using simple trigonometry, we can express d as d = a(5 + 2√5)*.

Beauty in Symmetry

Symmetry plays a significant role in the regular decagon. It possesses Dih10 symmetry, with an order of 20. This symmetry group offers fascinating subgroups, including Dih5, Dih2, and Dih1, and cyclic group symmetries like Z10, Z5, Z2, and Z1. Each subgroup symmetry allows for different degrees of freedom in irregular forms.

The regular decagon's highest symmetry irregular decagons are d10, an isogonal decagon, and p10, an isotoxal decagon. These two forms are duals of each other and have half the symmetry order of the regular decagon.

Expanding Horizons

Beyond the regular decagon, there are other exciting variations to explore. For example, the skew decagon, which consists of zig-zagging edges, can be seen in the vertices and side edges of a pentagonal antiprism, pentagrammic antiprism, and pentagrammic crossed-antiprism.

Moreover, the golden ratio, denoted by Φ, plays a significant role in both the construction and division of the regular decagon. It divides various line segments by exterior division, imparting a sense of harmony and balance to the shape.


The decagon is more than just a polygon with ten sides; it is a testament to the beauty and intricacy of geometry. Through its regular and irregular forms, the decagon showcases mesmerizing symmetries, elegant proportions, and intriguing mathematical relationships. Its allure and timeless appeal make it an object of fascination for mathematicians, artists, and enthusiasts alike.

So, the next time you come across a decagon, take a moment to appreciate its perfection and the wonders it holds within its ten sides.