Pentagon

In Geometry, we study about different types of shapes. The two-dimensional shape that is composed of straight lines and interior angles is known as a polygon. The examples of polygons are

  • Triangle ( three-sided polygon)
  • Quadrilateral ( four-sided polygon)
  • Pentagon ( five-sided polygon)
  • Hexagon ( six-sided polygon)
  • Heptagon ( seven-sided polygon)
  • Octagon ( eight-sided polygon) and so on.

Types of polygons

In this article, let us have a look at the five-sided polygon called “pentagon” with proper definition, shape, sides, properties along with its perimeter and area of a pentagon formula in detail.

Pentagon Definition

Pentagon

A pentagon is a polygon with 5 sides and 5 angles. The word “pentagon” is made up of two words, namely Penta and Gonia, which means five angles. All the sides of a pentagon meet with each other end to end to form a shape. Therefore,

The number of pentagon sides = 5

Pentagon shape

As other polygons like a triangle, quadrilaterals like a square, rectangle, etc., the pentagon is also a polygon that contains five sides and five angles.

Depending on the sides, angles, and vertices, there are different types of pentagon shapes, such as

  • Regular and Irregular pentagon
  • Convex and Concave pentagon

Regular and Irregular Pentagon

If a pentagon is regular, then all the sides are equal in length, and five angles are of equal measures. If the pentagon does not have equal side length and angle measure, then it is known as an irregular pentagon.

Regular and Irregular Pentagon

Convex and Concave Pentagon

If all the vertices of a pentagon are pointing outwards, it is known as a convex pentagon. If a pentagon has at least one vertex pointing inside, then the pentagon is known as a concave pentagon.

Convex and Concave Pentagon

Pentagon Properties

Some properties of the pentagon are as follows:

  • In the pentagon, the sum of the interior angles is equal to 540°.
  • If all the sides are equal and all the angles are of equal measure, then it is a regular polygon. Otherwise, it is irregular.
  • In the regular pentagon, the interior angle is 108°, and the exterior angle is of 72°.
  • An equilateral pentagon has 5 equal sides.
  • The sum of the interior angles of a rectangular pentagon is 540°.

Area of a Pentagon

Area of a Pentagon

For a regular pentagon with side and apothem length, then the formula to find the area of a pentagon is given as

Area of a Pentagon, A = (5/2) ×Side Length ×Apothem square units

If only the side length of a pentagon is given, then

Area = 5s2 / 4 tan 36° Square units

If only the radius of a pentagon is given, then

Area =(5/2)r2 sin 72° Square units

Perimeter of Pentagon

Since all the sides “a” of a regular pentagon are of equal measure, then the perimeter or circumference of a pentagon is written as,

The perimeter of a pentagon, P = 5a units

Pentagon Solved problem

Question:

Find the area and perimeter of a regular pentagon whose side is 5 cm and apothem length is 6 cm?

Solution:

Given:

The side of a pentagon, a = 5 cm

Apothem Length = 6 cm

We know that

The area of a pentagon, A = (5/2) ×Side Length ×Apothem square units

Substitute side = 5 cm, Apothem = 7 cm in formula,

A = (5/2) × 5 × 6

A = 5 × 5 × 3

A = 75

Therefore, the area of a pentagon is 75 cm2

The perimeter of a pentagon, P = 5a units

P = 5(5)

P = 25 cm

Hence, the perimeter of a pentagon is 25 cm.

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