<img src="https://d5nxst8fruw4z.cloudfront.net/atrk.gif?account=iA1Pi1a8Dy00ym" style="display:none" height="1" width="1" alt="" />
Dismiss
Skip Navigation

9.14: Relation of Polygon Sides to Angles and Diagonals

Difficulty Level: At Grade Created by: CK-12
Atoms Practice
Estimated5 minsto complete
%
Progress
Practice Relation of Polygon Sides to Angles and Diagonals
 
 
 
MEMORY METER
This indicates how strong in your memory this concept is
Practice
Progress
Estimated5 minsto complete
%
Estimated5 minsto complete
%
Practice Now
MEMORY METER
This indicates how strong in your memory this concept is
Turn In

Let's Think About It

Bradley is helping design an amusement park. He sketches his designs by using regular polygons. He also uses regular polygons to sketch the tables that he wants to place in the picnic area. He sketches a table that has 6 sides. What is the sum of the interior angles?

In this concept, you will learn how to relate the sides of a polygon to angle measures.

Guidance

Polygons can be divided into triangles using diagonals. This becomes very helpful when you try to figure out the sum of the interior angles of a polygon other than a triangle or a quadrilateral.

The sum of the interior angles of a quadrilateral is \begin{align*}360^\circ\end{align*}360.  You can divide a quadrilateral into two triangles using diagonals. Each triangle is \begin{align*}180^\circ\end{align*}180, so the sum of the interior angles of a quadrilateral is \begin{align*}360^\circ\end{align*}360.

Here is one diagonal in the quadrilateral. 

A diagonal is a line segment in a polygon that joins two nonconsecutive vertices. Divide up the polygon using diagonals and figure out the sum of the interior angles.

Here is a hexagon that has been divided into triangles by the diagonals. You can see here that there are four triangles formed. If the sum of the interior angles of each triangle is equal to \begin{align*}180^\circ\end{align*}180, and you have four triangles, then the sum of the interior angles of a hexagon is:

\begin{align*}4(180) = 720^\circ\end{align*}4(180)=720

You can follow this same procedure with any other polygon.

If you do not have a picture of the polygon, you can use a formula that involves the number of sides in a polygon.

To better understand how this works, let’s look at a table that shows the number of triangles related to the number of sides in a polygon.

The biggest pattern to notice is that the number of triangles is 2 less than the number of sides. If you know that the sum of the interior angles of one triangle is equal to 180 degrees and if you know that there are three triangles in a polygon, then you can multiply the number of triangles by 180 and that will give you the sum of the interior angles.

Here is the formula.

\begin{align*}x =\end{align*}x= number of sides

\begin{align*}(x - 2)180 =\end{align*}(x2)180= sum of the interior angles

You can take the number of sides and use that as \begin{align*}x\end{align*}x.

Then solve for the sum of the interior angles.

Let’s look at an example.

What is the sum of the interior angles of a decagon?

A decagon has ten sides. That is your \begin{align*}x\end{align*}x measurement. Now let’s use the formula.

\begin{align*}(x - 2)180 & = (10 - 2)180 \\ 8(180) & = 1440^\circ\end{align*}(x2)1808(180)=(102)180=1440

The answer is that there are \begin{align*}1440^\circ\end{align*}1440 in a decagon.

Guided Practice

What is the sum of the interior angles of a regular nonagon?

First, write the number of sides that are in a nonagon.

9

Next, plug the number of sides in to the formula.

\begin{align*}(9 - 2)180\end{align*}(92)180

Then, solve for the sum of the interior angles.

 \begin{align*}(7)(180) = 1260\end{align*}(7)(180)=1260

The answer is \begin{align*}1260^\circ\end{align*}1260.

Examples

Calculate the sums for each example.

Example 1

The sum of the interior angles of a pentagon

First, write the number of sides that are in a pentagon.

5

Next, plug the number of sides in to the formula.

 \begin{align*}(5-2)(180)\end{align*}(52)(180)

Then, solve for the sum of the interior angles.

 \begin{align*}(3)(180) = 540\end{align*}(3)(180)=540

The answer is \begin{align*}540^o\end{align*}540o.

Example 2

The sum of the interior angles of a triangle

First, write the number of sides that are in a triangle.

3

Next, plug the number of sides in to the formula.

 \begin{align*}(3-2)(180)\end{align*}(32)(180)

Then, solve for the sum of the interior angles.

 \begin{align*}1(180) = 180\end{align*}1(180)=180

The answer is \begin{align*}180^o\end{align*}180o.

Example 3

The sum of the interior angles of an octagon

First, write the number of sides that are in an octagon.

8

Next, plug the number of sides in to the formula?'

 \begin{align*}(8-2)(180)\end{align*}(82)(180)

Then, solve for the sum of the interior angles.

 \begin{align*}6(180) = 1080\end{align*}6(180)=1080

The answer is \begin{align*}1080^o\end{align*}1080o.

Follow Up

Remember Bradley and his picnic table designs? He sketches a table that has 6 sides. What is the sum of the interior angles?

First, write the number of sides that are in the polygon.

6

Next, plug the number of sides in to the formula.

\begin{align*}(6-2)(180)\end{align*}(62)(180)\begin{align*}(5-2)(180)\end{align*}(52)(180)

Then, solve for the sum of the interior angles.

\begin{align*}4(180) = 720\end{align*}4(180)=720

The answer is \begin{align*}720^o\end{align*}720o. The sum of the interior angles is \begin{align*}720^o\end{align*}720o.

Video Review

Explore More

Look at each image and name the type of polygon pictured.

1.

2.

3.

4.

5.

6.

Name the number of diagonals in each polygon.

7.

8.

9.

Use the formula to name the sum of the interior angles of each polygon.

10. Hexagon

11. Pentagon

12. Decagon

13. Heptagon

14. Octagon

15. Square

Answers for Explore More Problems

To view the Explore More answers, open this PDF file and look for section 9.14. 

Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Please to create your own Highlights / Notes
Show More

Vocabulary

TermDefinition
Congruent Congruent figures are identical in size, shape and measure.
Decagon A decagon is a ten sided polygon.
Diagonal A diagonal is a line segment in a polygon that connects nonconsecutive vertices
Heptagon A heptagon is a seven sided polygon.
Hexagon A hexagon is a six sided polygon.
Irregular Polygon An irregular polygon is a polygon with non-congruent side lengths.
Nonagon A nonagon is a nine sided polygon.
Nonconsecutive Nonconsecutive means not next to each other or not in order.
Octagon An octagon is an eight sided polygon.
Pentagon A pentagon is a five sided polygon.
Polygon A polygon is a simple closed figure with at least three straight sides.
Regular Polygon A regular polygon is a polygon with all sides the same length and all angles the same measure.

Image Attributions

Show Hide Details
Description
Difficulty Level:
At Grade
Grades:
Date Created:
Oct 29, 2012
Last Modified:
Sep 04, 2016
Files can only be attached to the latest version of Modality
Please wait...
Please wait...
Image Detail
Sizes: Medium | Original
 
MAT.GEO.411.L.1