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9.13: Regular and Irregular Polygons

Difficulty Level: Basic Created by: CK-12
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Have you ever designed something and discovered an error in your plan? Well, look at what happened to Keith.

Keith began working on a design for a sign to advertise the opening of the skatepark. He didn't want to make it ordinary, so he chose a pentagon as the shape of the sign.

Keith drew it out and began to work on the lettering. When he was finished, he held up the sign and noticed that the bottom line of the pentagon was a different length from the other lines. The other four lines of the pentagon were all 2 feet in length, while the bottom line measured 2.5 feet.

Is this still a pentagon? Why?

This Concept is all about regular and irregular polygons. At the end of the Concept, you will know how to identify the pentagon that Keith has drawn.

Guidance

Now that you have been introduced to the different types of polygons, it is time to learn about classifying polygons. All polygons can be classified as regular or irregular polygons. You have to understand the difference between a regular or irregular polygon to classify each shape. Let’s learn how we can tell the difference between them.

What is a regular polygon?

A regular polygon is a polygon where all of the side lengths are equal. In other words, the polygon is an equilateral polygon where all the side lengths are congruent .

This triangle is a regular triangle. All three side lengths are congruent. Here is an example of an irregular polygon.

By counting the sides, you can see that this is a five sided figure. It is a pentagon. However, the sides are not congruent. Therefore, it is an irregular pentagon.

Irregular polygons have side lengths that are not congruent.

Now it’s time for you to practice. Name each figure as a regular or irregular polygon.

Example A

Solution: Regular Polygon

Example B

Solution: Irregular Polygon

Example C

An octagon with all sides measuring 4 centimeters.

Solution: Regular polygon

Here is the original problem once again.

Keith began working on a design for a sign to advertise the opening of the skatepark. He didn't want to make it ordinary, so he chose a pentagon as the shape of the sign.

Keith drew it out and began to work on the lettering. When he was finished, he held up the sign and noticed that the bottom line of the pentagon was a different length from the other lines. The other four lines of the pentagon were all 2 feet in length, while the bottom line measured 2.5 feet.

Is this still a pentagon? Why?

This figure is still a pentagon, but it is not a regular pentagon.

To be a regular pentagon, all of the side lengths would need to be equal in length.

Because one side is longer than the other four, this is an irregular pentagon.

However, it still has five sides, so it is still considered a pentagon.

Vocabulary

Here are the vocabulary words in this Concept.

Polygon
A simple closed figure formed by three or more line segments.
Pentagon
five sided polygon
Hexagon
six sided polygon
Heptagon
seven sided polygon
Octagon
eight sided polygon
Nonagon
nine sided polygon
Decagon
ten sided polygon
Regular Polygon
polygon with all sides congruent
Irregular Polygon
a polygon where all of the side lengths are not congruent
Congruent
exactly the same or equal

Guided Practice

Here is one for you to try on your own.

Jessie's new bedroom is square in shape. She measures one of the walls and finds that it is 12 feet long. She measures another wall and finds that it is 10 feet long. The other two walls are also different lengths.

Is Jessie's room a square?

Is it a regular polygon or an irregular polygon?

Answer

Jessie's room can not be a square because all of the walls are different lengths. It is also irregular in shape for this reason. Jessie's room is a quadrilateral because it has four walls, but is not a regular polygon.

Video Review

Here are videos for review.

James Sousa, Introduction to Polygons

James Sousa, Classifying Polygons

Practice

Directions : Based on each description, determine whether the polygon is regular or irregular.

1. A quadrilateral with all sides equal to 5 cm.

2. A pentagon with different side lengths.

3. A square.

4. A rectangle.

5. An octagon with one side that is 8 cm and the other sides congruent to that side.

6. A heptagon with seven congruent sides and angles.

7. A hexagon with six congruent angles.

8. A hexagon with the following side lengths: 4 m, 5 m, 7 m, 5 m, 4 m and 7 m.

9. A pentagon with five congruent angles.

10. True or false. A figure can be a polygon and be regular or irregular.

Directions : Identify each polygon below.

11. a

12. b

13. c

14. d

Directions : Now tell whether each is regular or irregular.

15. a

16. b

17. c

18. d

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Difficulty Level:

Basic

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Grades:

Date Created:

Oct 29, 2012

Last Modified:

Aug 18, 2014
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