In geometry, a decagon is known as a ten-sided polygon or ten-gon. The sum of the interior angles of a simple decagon is 1440° and the sum of the exterior angles of a decagon is 360°.

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1.What is a Decagon?
2.Types of Decagon
3.Properties of Decagon
4.Decagon Formulas
5.Decagon Examples
6.Practice Questions
7.FAQs on Decagon

A decagon is a ten-sided polygon with ten vertices and ten angles.Thus, a decagon shape can be defined as a polygon having ten sides, ten interior angles, and ten vertices. Based on the sides of a decagon, they are broadly classified into regular decagons and irregular decagons. A regular decagon has 35 diagonals and 8 triangles. The location of these diagonals and triangles is explained in the later sections of this article.


Decagons can be categorized as regular and irregular decagons based on side-length and angle measurements. There are three possible classifications of decagon that are given below:

Regular and Irregular DecagonsConvex and Concave DecagonsSimple and Complex Decagons

Regular Decagon

A regular decagon is a polygon along with10 equal sides and 10 vertices.The sides and angles are congruent in a regular decagon. The characteristics of a regular decagon are:

Each interior angle in regular decagon measures 144º, while each exterior angle measures 36º.

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Irregular Decagon

An irregular decagon does not have equal sides and angles. At least two sides and angles are different in measurement. Look at the images given below showing irregular decagons.

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Convex and Concave Decagons

Like any other polygon, decagons also can be convex and concave. A convex decagon bulges outward as all the interior angles are lesser than 180°. While concave decagons have indentations (a deep recess). At least one of the interior angles is greater than 180° in concave decagons.

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Simple and Complex Decagons

Simple decagons refer to decagons with no sides crossing themselves. They follow all of the above said regular decagon rules. While complex decagons refer to decagons that are self-intersecting and have additional interior spaces. They do not strictly follow any prescribed rules of decagons regarding their interior angles and their sums.

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Properties of Decagon


Some of the important properties of decagons are listed here.

The sum of the interior angles is 1440°.The sum of the measurements of the exterior angle is 360°.The central angle measures 36 degrees in the case of a regular decagon.There are 35 diagonals in a decagon.There are 8 triangles in a decagon.

Sum of the Interior Angles of Decagon

To find the sum of the interior angles of a decagon, first, divide it into triangles. There are eight triangles in a regular decagon. We know that the sum of the angles in each triangle is 180°. Thus,180° × 8 = 1440°. Therefore, the sum of all the interior angles of a decagon is1440°.We know that the number of sides of a decagon is 10. Hence, we divide the total sum of the interior angles by 101440° ÷ 10 = 144°Thus, one interior angle of a regular decagon shape is 144°. And, the sum of all the interior angles of a decagon is 1440°.

Measure of the Central Angles of a Regular Decagon

To find the measure of the central angle of a regular decagon, we need to drawa circle in the middle. A circle forms 360°.Divide this by ten, because a decagonhas 10 sides. 360° ÷ 10 = 36°. Thus, the measure of the central angle of a regular decagon is36°.

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Decagon Diagonals

A diagonal is a line that can be drawn from one vertexto another. The number of diagonals of a polygon is calculated by:n(n−3) ÷ 2. In decagon, n is the number of sides which is equal to 10, so n=10. Now we get,n(n−3) ÷ 2 = 10(10−3) ÷ 2Thus, the number of diagonals in a decagon is 35.

Decagon has 8 Triangles

By joining one vertex to the remaining vertices of the decagon,8 triangles will be formed. By joiningall the vertices independently to each other, then 80 triangles (8×10) will be formed. Look at the image given below, showing diagonals and triangles of a decagon.

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Decagon Formulas


Like other shapes, a regular decagon also has the formula to calculateperimeter andarea. The formulas are mentioned below:

The formula to find the area of a decagon is (dfrac5a^22 imessqrt5 + 2sqrt5),where ais the measurement of the side-length of the decagon.

The formula to find the perimeter of a regular decagon is 10 times of a side or 10 × n, where n is the side-length of the decagon (as all sides are equal and total sides are 10). In the case of an irregular decagon, we can simply add the side lengths to find the perimeter.

Important Notes

A decagon has ten sides.The sum of the interior angles of a decagon is1440°.The sum of the exterior angles of a decagon is360°.A regular decagon has 35 diagonals.Decagons can be classified as regular, irregular, convex, concave, simple, and complex.

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