Area of Decagon
The area of a decagon is defined as the number of unit squares that can be fit within it. We will discuss in this lesson the concept of the area of a decagon. Decagons as shapes are around us in the form of coins, watches, designs, and patterns. A decagon is a 2dimensional tensided polygon. The word is made up of "deca" and "gon" where "deca" means ten and "gon" means sides. In this lesson, we will learn to determine the area of a decagon using examples. Stay tuned to learn more!!!
1.  What is the Area of Decagon? 
2.  How to Find the Area of Decagon? 
3.  What is the Formula for Area of Decagon? 
4.  FAQs on Area of Decogon 
What is Area of Decagon?
The area of the decagon is the amount of region it covers. A decagon is a plane figure with 10 sides. It also has 10 interior angles. A regular decagon has all equal sides and all equal interior angles. So, for a regular decagon with 10 sides, we can draw 20 diagonals and it has 10 vertices. Since the sum of all the interior angles of a decagon is 1440°, the value of each interior angle for a regular decagon is 144°. The sum of all the exterior angles of a regular decagon is 360°. The unit of area of decagon can be given in terms of m^{2}, cm^{2}, in^{2} or ft^{2}.
How to Find the Area of Decagon?
A regular decagon is divided into 10 congruent isosceles triangles when all its diagonals are drawn. Therefore the area of a decagon is given as area of a decagon = area of 10 congruent isosceles triangles so formed
⇒ Area of a decagon = 10 × Area of each congruent isosceles triangle
We can find the area of a decagon using the following steps:
 Step 1: Find the area of each congruent isosceles triangle.
 Step 2: Multiply the value of the area of each congruent isosceles triangle by 10.
 Step 3: Write the unit in the end, once the value is obtained.
What is the Formula of the Area of Decagon?
Let's use the fact that the area of a decagon is equal to the area of the 10 isosceles triangles formed in a decagon when diagonals are drawn. Therefore,
⇒ Area of a decagon = 10 × Area of each congruent isosceles triangle
Let's first find the area of each isosceles tringle first:
Area of each isosceles triangle = 1/2 × Base × Height
⇒ Area of each isosceles triangle = 1/2 × a × h
Height of the isosceles triangle, h = a/2 × tan 72° = a/2 × \({ \ \sqrt{10+2\sqrt{5}} \over \sqrt{5}1}\)
⇒ Area of each isosceles triangle = 1/2 × a × a/2 × \({ \ \sqrt{10+2\sqrt{5}} \over \sqrt{5}1}\)
⇒ Area of each isosceles triangle = a^{2}/4 × \({ \ \sqrt{10+2\sqrt{5}} \over \sqrt{5}1}\)
Substituting the value, we get:
Area of a decagon = 10 × a^{2}/4 × \({ \ \sqrt{10+2\sqrt{5}} \over \sqrt{5}1}\)
⇒ Area of a decagon = 5a^{2}/2 × \({\sqrt {20 + 8\sqrt{5} \over 4}}\)
= 5a^{2}/2 × \({\sqrt {5 + 2\sqrt{5}}}\)
Therefore, the formula for the area of a decagon is 5a^{2}/2 × \({\sqrt {5 + 2\sqrt{5}}}\). So if we know the value of the side length of a regular octagon we can easily find its area.
Solved Examples on Area of Decagon

Example 1: Find the area of a regular decagon with a side length of 10 cm.
Solution: Given, the side length of a decagon, a = 10 cm
As we know, the area of the decagon = 5a^{2}/2 × \({\sqrt {5 + 2\sqrt{5}}}\)
⇒ Area of a decagon = 5 × 10^{2}/2 × \({\sqrt {5 + 2\sqrt{5}}}\)
⇒ Area of a decagon = 250 × \({\sqrt {5 + 2\sqrt{5}}}\) ≈ 769.42 square units.
Therefore, the area of the regular decagon is 769.42 square units. 
Example 2: Find the area of a regular decagon if the area of one of the isosceles triangles formed by its diagonals is 9 square units.
Solution: Given, the area of one of the isosceles triangles formed by diagonal is 9 square units.
As we know, area of a decagon = 10 × area of each congruent isosceles triangle
⇒ Area of the decagon = 10 × 9
⇒ Area of a decagon = 90 square units
Therefore the area of the decagon is 90 square units.
FAQs on Area of a Decagon
How Many Sides Are There in a Decagon?
The word decagon is made of two words "deca" and "gon", where the word "deca" refers to ten and "gon" refers to sides. Thus, there are ten sides in a decagon.
How Can We Find the Area of a Decagon?
We can find the area of the decagon using the following steps:
 Step 1: Detemine the area of each congruent isosceles triangle.
 Step 2: Now, multiply the value of the area of each congruent isosceles triangle by 10.
 Step 3: Once the value of the area of the decagon is obtained, write the unit in the end.
What is the Formula to Find the Area of a Regular Decagon?
The formula to determine the value of the area of a decagon is 5a^{2}/2 × \({\sqrt {5 + 2\sqrt{5}}}\), where 'a' is the side length of the decagon.
What Are the Units Used for the Area of a Decagon?
The unit of 'area' is "square units". For example, it can be expressed as m^{2}, cm^{2}, in^{2}, etc depending upon the given units.
How to Find the Area of Decagon If the Area of an Isosceles Triangle Formed By Its Diagonal is Known?
We can determine the area of decagon if the area of an isosceles formed by its diagonal is known using the following steps:
 Step 1: Write the dimension of the area of an isosceles triangle formed.
 Step 2: Determine the area of decagon using the formula area of a decagon = 10 × area of each congruent isosceles triangle.
 Step 3: Once the value of the area of the decagon is obtained, write the unit in the end.
What Happens to the Area of Regular Decagon If the Length of the Side of Decagon is Doubled?
The area of a regular decagon quadruples if the length of the side of the decagon is doubled as "a" in the formula gets substituted by "2a". Thus, A = 5(2a)^{2}/2 × \({\sqrt {5 + 2\sqrt{5}}}\) = 4 (5a^{2}/2 × \({\sqrt {5 + 2\sqrt{5}}}\)) which gives four times the original value of the area.
What Happens to the Area of Regular Decagon If the Length of the Side of Decagon is Halved?
The area of a regular decagon becomes onefourth if the length of the side of the decagon is halved as "a" in the formula gets substituted by "a/2". Thus, A = 5(a/2)^{2}/2 × \({\sqrt {5 + 2\sqrt{5}}}\) = (1/4) × (5a^{2}/2 × \({\sqrt {5 + 2\sqrt{5}}}\)) which gives onefourth the original value of the area.