Perimeter and Area of a Pentagon – Formulas and Examples

The perimeter of a pentagon is the length of the outline of the pentagon. On the other hand, its area represents the two-dimensional space occupied by the figure. We can find the perimeter of a pentagon by adding the lengths of its five sides, and we can find its area by multiplying the length of the apothem by the length of one of its sides and multiplying by 5/2.

In this article, we’ll explore all things related to the perimeter and area of a pentagon. We will explore its formulas and apply them to solve some practice problems.

GEOMETRY
Formulas for the perimeter and area of a pentagon

Relevant for

Learning about the perimeter and area of a pentagon.

See examples

GEOMETRY
Formulas for the perimeter and area of a pentagon

Relevant for

Learning about the perimeter and area of a pentagon.

See examples

How to find the perimeter of a pentagon?

The perimeter of a pentagon represents the total length of the pentagon’s boundaries. Therefore, we can calculate the perimeter of a pentagon by adding the lengths of its five sides:

$latex p=a+b+c+d+e$

where, $latex a,~b,~c,~d,~e$ are the lengths of the sides of the pentagon.

If the pentagon is regular, all five of its sides have the same length, so the formula for the perimeter becomes:

$latex p=5a$

where, a is the length of one of the sides of the pentagon.

diagram of a pentagon with all the sides

How to find the area of a pentagon?

The area of a regular pentagon can be calculated using the length of its apothem and the length of one of its sides. Therefore, we can use the following formula:

$latex A= \frac{5}{2} \times s \times a$

where s is the length of one of the sides and a is the length of the apothem. Let us remember that the apothem is the segment that joins the center of the pentagon with one of its sides.

Alternatively, we can calculate the area of a pentagon using the following formula:

$latex A=\frac{1}{4}\sqrt{5(5+2\sqrt{5}){{s}^2}}$

where, s is the length of one of the sides of the pentagon. This formula is more complicated, but it allows us to calculate the area of a regular pentagon simply with the length of one of its sides.

Proof of the formula for the area of a pentagon

To prove the formula for the area of a pentagon, we are going to use the following diagram, where we divide the pentagon into five isosceles triangles as in the following image:

diagram of a pentagon with apothem

The area of any triangle can be calculated using the formula $latex A= \frac{1}{2} bh$, where b is the base of the triangle and h is its height. In the isosceles triangles above, the base is equal to the length of one of the sides of the pentagon (length s.)

The height of the triangle is equal to the apothem of the pentagon (length a). Therefore, the area of each triangle in the pentagon is $latex A= \frac{1}{2} la$. Since we have five triangles, the area of the pentagon is:

$latex A= \frac{5}{2} \times s \times a$


Perimeter and area of a pentagon – Examples with answers

The following examples apply the formulas for the perimeter and area of a pentagon. Each example has its solution, but try to solve the problems yourself before looking at the answer.

EXAMPLE 1

Find the perimeter of a regular pentagon that has sides with a length of 10 inches.

We are going to use the perimeter formula with the value $latex a=10$. Therefore, we have:

$latex p=5a$

$latex p=5(10)$

$latex p=50$

The perimeter of the pentagon is equal to 50 in.

EXAMPLE 2

Find the area of a regular pentagon that has sides with a length of 10 inches and an apothem with a length of 6.88 in.

We have the following lengths:

  • Sides, $latex s=10$ in
  • Apothem, $latex a=6.88$ in

Substituting these values into the formula for the area of a pentagon, we have:

$latex A= \frac{5}{2}sa$

$latex A= \frac{5}{2}(10)(6.88)$

$latex A=172$

The area of the pentagon is equal to 172 in².

EXAMPLE 3

Find the perimeter of a regular pentagon that has sides with a length of 11 feet.

We use the length $latex a=11$ in the formula for the perimeter, and we have:

$latex p=5a$

$latex p=5(11)$

$latex p=55$

The perimeter of the pentagon is equal to 55 ft.

EXAMPLE 4

What is the area of a regular pentagon that has sides with a length of 8 inches and an apothem with a length of 5.51 inches?

We have the following lengths:

  • Side, $latex s=8$ in
  • Apothem, $latex a=5.51$ in

Applying the formula for the area with these lengths, we have:

$latex A= \frac{5}{2}sa$

$latex A= \frac{5}{2}(8)(5.51)$

$latex A=110.2$

The area of the pentagon is equal to 110.2 in².

EXAMPLE 5

What is the perimeter of a regular pentagon that has sides with a length of 15 yards?

We use the formula for the perimeter with the length $latex a=15$. Therefore, we have:

$latex p=5a$

$latex p=5(15)$

$latex p=75$

The perimeter of the pentagon is equal to 75 yd.

EXAMPLE 6

Find the apothem of a regular pentagon that has an area of 84.3 ft² and the length of its sides is 7 ft.

We have the following:

  • Area, $latex A=84.3$ ft²
  • Sides, $latex s=7$ ft

In this case, we know the area of the pentagon, and we want to find the apothem. Therefore, we use the information given in the formula for the area and solve for a:

$latex A= \frac{5}{2}sa$

$latex 84.3= \frac{5}{2}(7)a$

$latex 168.6=(5)(7)a$

$latex 168.6=35a$

$latex a=4.82$

The length of the apothem is 4.82 ft.

EXAMPLE 7

If the perimeter of a regular pentagon is equal to 20 yards, what is the length of one of its sides?

Here, we know the perimeter of the pentagon, and we need to find the length of one of the sides. Therefore, we use the perimeter formula and solve for a:

$latex p=5a$

$latex 20=5a$

$latex a=4$

The length of one side of the pentagon is 4 yd.

EXAMPLE 8

Find the area of a regular pentagon that has sides with a length of 5 inches.

In this case, we only know the length of the sides of the pentagon and not its apothem. Therefore, we can use the second formula for the area of a pentagon:

$latex A= \frac{1}{4}\sqrt{5(5+2\sqrt{5}){{s}^2}}$

$latex A= \frac{1}{4}\sqrt{5(5+2\sqrt{5}){{(5)}^2}}$

$latex A=43.01$

The area of the pentagon is equal to 43.01 in².

EXAMPLE 9

If the perimeter of a regular pentagon is equal to 105 feet, find the length of one of its sides.

Let’s use the formula for the perimeter and solve for a. Therefore, using the value $latex p=105$, we have:

$latex p=5a$

$latex 105=5a$

$latex a=21$

The length of one of the sides is 21 ft.

EXAMPLE 10

Find the area of a regular pentagon that has sides with a length of 20 inches.

Using the second formula for the area with the length $latex s=20$:

$latex A= \frac{1}{4}\sqrt{5(5+2\sqrt{5}){{s}^2}}$

$latex A= \frac{1}{4}\sqrt{5(5+2\sqrt{5}){{(20)}^2}}$

$latex A=688.19$

The area of the pentagon is equal to 688.19 in².


Perimeter and area of a pentagon – Examples with answers

Solve the following problems by applying everything learned about the perimeter and the area of pentagons. You can use the solved examples above as a guide.

What is the perimeter of a regular pentagon that has sides with a length of 9 yards?

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Find the area of a pentagon that has sides with a length of 6 feet and an apothem of 4.13 feet.

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Find the perimeter of a regular pentagon that has sides with a length of 14 in.

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Find the area of a pentagon that has sides with a length of 11 inches and an apothem of 7.57 inches.

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Find the length of the sides of a regular pentagon that has a perimeter of 35 yards.

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Find the area of a regular pentagon that has sides of length 4 feet.

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