4.4 Solve Geometry Applications: Circles and Irregular Figures

Pooja Gupta

4.4 Solve Geometry Applications: Circles and Irregular Figures

Learning Objectives

By the end of this section, you will be able to:

Use the properties of circles
Find the area of irregular figures

In this section, we’ll continue working with geometry applications. We will add several new formulas to our collection of formulas. To help you as you do the examples and exercises in this section, we will show the Problem Solving Strategy for Geometry Applications here.

Problem Solving Strategy for Geometry Applications

Read the problem and make sure you understand all the words and ideas. Draw the figure and label it with the given information.
Identify what you are looking for.
Name what you are looking for. Choose a variable to represent that quantity.
Translate into an equation by writing the appropriate formula or model for the situation. Substitute in the given information.
Solve the equation using good algebra techniques.
Check the answer in the problem and make sure it makes sense.
Answer the question with a complete sentence.

Use the Properties of Circles

We’ll refer to the properties of circles as we use them to solve applications.

Properties of Circles

$\bullet \quad r$ is the length of the radius
$\bullet \quad d$ is the length of the diametre
$d=2r$
$\bullet$ Circumference is the perimeter of a
circle. The formula for circumference is $C=2\pi r$
$\bullet$ The formula for area of a circle is $A=\pi {r}^{2}$

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Remember, that we approximate $\pi$ with $3.14$ or $\frac{22}{7}$ depending on whether the radius of the circle is given as a decimal or a fraction. If you use the $\pi$ key on your calculator to do the calculations in this section, your answers will be slightly different from the answers shown. That is because the $\pi$ key uses more than two decimal places.

EXAMPLE 1

A circular sandbox has a radius of $2.5$ feet. Find the a) circumference and b) area of the sandbox.

Solution

a)

Step 1. Read the problem. Draw the figure and label it with the given information.
Step 2. Identify what you are looking for.	the circumference of the circle
Step 3. Name. Choose a variable to represent it.	Let c = circumference of the circle
Step 4. Translate. Write the appropriate formula Substitute	$C=2\pi r$ $C=2\pi \left(2.5\right)$
Step 5. Solve the equation.	$C\approx 2\left(3.14\right)\left(2.5\right)$ $C\approx 15.7\text{ft}$
Step 6. Check. Does this answer make sense? Yes. If we draw a square around the circle, its sides would be 5 ft (twice the radius), so its perimeter would be 20 ft. This is slightly more than the circle’s circumference, 15.7 ft.
Step 7. Answer the question.	The circumference of the sandbox is 15.7 feet.

b)

Step 1. Read the problem. Draw the figure and label it with the given information
Step 2. Identify what you are looking for.	the area of the circle
Step 3. Name. Choose a variable to represent it.	Let A = the area of the circle
Step 4. Translate. Write the appropriate formula Substitute	$A=\pi{r}^{2}$ $A=\pi{\left(2.5\right)}^{2}$
Step 5. Solve the equation.	$A\approx \left(3.14\right){\left(2.5\right)}^{2}$ $A\approx 19.625\phantom{\rule{0.2em}{0ex}}\text{sq. ft}$
Step 6. Check. Yes. If we draw a square around the circle, its sides would be 5 ft, as shown in part a). So the area of the square would be 25 sq. ft. This is slightly more than the circle’s area, 19.625 sq. ft.
Step 7. Answer the question.	The area of the circle is 19.625 square feet.

TRY IT 1.1

A circular mirror has radius of $5$ inches. Find the a) circumference and b) area of the mirror.

Answer

31.4 in.
78.5 sq. in.

TRY IT 1.2

A circular spa has radius of $4.5$ feet. Find the a) circumference and b) area of the spa.

Answer

28.26 ft
63.585 sq. ft

We usually see the formula for circumference in terms of the radius $r$ of the circle:

$C=2\pi r$

But since the diametre of a circle is two times the radius, we could write the formula for the circumference in terms $\text{of}\phantom{\rule{0.2em}{0ex}}d$ .

$\begin{array}{cccc}& & & C=2\pi r\hfill \\ \text{Using the commutative property, we get}\hfill & & & C=\pi \cdot2r\hfill \\ \text{Then substituting}\phantom{\rule{0.2em}{0ex}}d=2r\hfill & & & C=\pi \cdot\text{d}\hfill \\ \text{So}\hfill & & & C=\pi d\hfill \end{array}$

We will use this form of the circumference when we’re given the length of the diametre instead of the radius.

EXAMPLE 2

A circular table has a diametre of four feet. What is the circumference of the table?

Solution

Step 1. Read the problem. Draw the figure and label it with the given information.
Step 2. Identify what you are looking for.	the circumference of the table
Step 3. Name. Choose a variable to represent it.	Let c = the circumference of the table
Step 4. Translate. Write the appropriate formula for the situation. Substitute.	$C=\pi d$ $C=\pi \left(4\right)$
Step 5. Solve the equation, using 3.14 for $\pi$ .	$C\approx \left(3.14\right)\left(4\right)$ $C\approx 12.56\phantom{\rule{0.2em}{0ex}}\text{ sq. feet}$
Step 6. Check: If we put a square around the circle, its side would be 4. The perimeter would be 16. It makes sense that the circumference of the circle, 12.56, is a little less than 16.
Step 7. Answer the question.	The diametre of the table is 12.56 square feet

TRY IT 2.1

Find the circumference of a circular fire pit whose diametre is $5.5$ feet.

Answer

17.27 ft

TRY IT 2.2

If the diametre of a circular trampoline is $12$ feet, what is its circumference?

Answer

37.68 ft

EXAMPLE 3

Find the diametre of a circle with a circumference of $47.1$ centimetres.

Solution

Step 1. Read the problem. Draw the figure and label it with the given information.
Step 2. Identify what you are looking for.	the diametre of the circle
Step 3. Name. Choose a variable to represent it.	Let d = the diametre of the circle
Step 4. Translate.
Write the formula. Substitute, using 3.14 to approximate $\pi$ .	$\begin{aligned} C&=\pi d \\ 47.1 & \approx 3.14d \end{aligned}$
Step 5. Solve.	$\begin{aligned} \frac{47.1}{3.14} &\approx \frac{3.14 d}{3.14} \\ 15 &\approx d \end{aligned}$
Step 6. Check:	$C=\pi d$ $47.1\stackrel{?}{=}\left(3.14\right)\left(15\right)$ $47.1=47.1$ ✓
Step 7. Answer the question.	The diametre of the circle is approximately 15 centimetres.

TRY IT 3.1

Find the diametre of a circle with circumference of $94.2$ centimetres.

Answer

30 cm

TRY IT 3.2

Find the diametre of a circle with circumference of $345.4$ feet.

Answer

110 ft

Find the Area of Irregular Figures

So far, we have found area for rectangles, triangles, trapezoids, and circles. An irregular figure is a figure that is not a standard geometric shape. Its area cannot be calculated using any of the standard area formulas. But some irregular figures are made up of two or more standard geometric shapes. To find the area of one of these irregular figures, we can split it into figures whose formulas we know and then add the areas of the figures.

EXAMPLE 4

Find the area of the shaded region.

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Solution

The given figure is irregular, but we can break it into two rectangles. The area of the shaded region will be the sum of the areas of both rectangles.

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The blue rectangle has a width of $12$ and a length of $4$ . The red rectangle has a width of $2$ , but its length is not labeled. The right side of the figure is the length of the red rectangle plus the length of the blue rectangle. Since the right side of the blue rectangle is $4$ units long, the length of the red rectangle must be $6$ units.

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$\begin{aligned} & A_{\text {figure }}=A_{\text {\textcolor{blue}{rectangle} }}+A_{\text {\textcolor{red}{rectangle} }} \\ & A_{\text {figure }}={\color{blue}{b h}}+{\color{red}{b h}} \\ & A_{\text {figure }}=12 \cdot 4+2 \cdot 6 \\ & A_{\text {figure }}=48+12 \\ & A_{\text {figure }}=60\end{aligned}$

The area of the figure is $60$ square units.

Is there another way to split this figure into two rectangles? Try it, and make sure you get the same area.

TRY IT 4.1

Find the area of each shaded region:

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Answer

28 sq. units

TRY IT 4.2

Find the area of each shaded region:

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Answer

110 sq. units

EXAMPLE 5

Find the area of the shaded region.

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Solution

We can break this irregular figure into a triangle and rectangle. The area of the figure will be the sum of the areas of triangle and rectangle.

The rectangle has a length of $8$ units and a width of $4$ units.

We need to find the base and height of the triangle.

Since both sides of the rectangle are $4$ , the vertical side of the triangle is $3$ , which is $7-4$ .

The length of the rectangle is $8$ , so the base of the triangle will be $3$ , which is $8-4$ .

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Now we can add the areas to find the area of the irregular figure.

$\begin{aligned} & A_{\text {figure }}=A_{\color{blue}{\text {rectangle }}}+A_{\color{red}{\text {triangle }}} \\ & A_{\text {figure }}={\color{blue}{lw}}+{\color{red}{\frac{1}{2} b h} }\\ & A_{\text {figure }}={\color{blue}8 \cdot 4}+{\color{red}{\frac{1}{2} \cdot 3 \cdot 3}} \\ & A_{\text {figure }}={\color{blue}32}+\color{red}{4.5} \\ & A_{\text {figure }}=36.5 \text { sq. units } \end{aligned}$

The area of the figure is $36.5$ square units.

TRY IT 5.1

Find the area of each shaded region.

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Answer

36.5 sq. units

TRY IT 5.2

Find the area of each shaded region.

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Answer

70 sq. units

EXAMPLE 6

A high school track is shaped like a rectangle with a semi-circle (half a circle) on each end. The rectangle has length $105$ metres and width $68$ metres. Find the area enclosed by the track. Round your answer to the nearest hundredth.

A track is shown, shaped like a rectangle with a semi-circle attached to each side.

Solution

We will break the figure into a rectangle and two semi-circles. The area of the figure will be the sum of the areas of the rectangle and the semicircles.

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The rectangle has a length of $105$ m and a width of $68$ m. The semi-circles have a diametre of $68$ m, so each has a radius of $34$ m.

$\begin{aligned} & A_{\text {figure }}=A_{\color{blue}{\text {rectangle }}}+A_{\color{red}{\text {semicircles }} }\\ & A_{\text {figure }}={\color{blue}b h}+{\color{red}2\left(\frac{1}{2} \pi \cdot r^2\right)} \\ & A_{\text {figure }} \approx {\color{blue}105 \cdot 68}+{\color{red}2\left(\frac{1}{2} \cdot 3.14 \cdot 34^2\right) }\\ & A_{\text {figure }} \approx {\color{blue}7140}+ {\color{red}3629.84} \\ & A_{\text {figure }} \approx 10,769.84 \text { square meters } \end{aligned}$

TRY IT 6.1

Find the area of the shaded shape:

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Answer

103.2 sq. units

TRY IT 6.2

Find the area:

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Answer

38.24 sq. units

Access Additional Online Resources

Key Concepts

Problem Solving Strategy for Geometry Applications
1. Read the problem and make sure you understand all the words and ideas. Draw the figure and label it with the given information.
2. Identify what you are looking for.
3. Name what you are looking for. Choose a variable to represent that quantity.
4. Translate into an equation by writing the appropriate formula or model for the situation. Substitute in the given information.
5. Solve the equation using good algebra techniques.
6. Check the answer in the problem and make sure it makes sense.
7. Answer the question with a complete sentence.
Properties of Circles

$d=2r$
Circumference: $C=2\pi r$ or $C=\pi d$
Area: $A=\pi {r}^{2}$

Glossary

irregular figure: An irregular figure is a figure that is not a standard geometric shape. Its area cannot be calculated using any of the standard area formulas.

Practice Makes Perfect

Use the Properties of Circles

In the following exercises, solve using the properties of circles.

1. The lid of a paint bucket is a circle with radius $7$ inches. Find the a) circumference and b) area of the lid.	2. An extra-large pizza is a circle with radius $8$ inches. Find the a) circumference and b) area of the pizza.
3. A farm sprinkler spreads water in a circle with radius of $8.5$ feet. Find the a) circumference and b) area of the watered circle.	4. A circular rug has radius of $3.5$ feet. Find the a) circumference and b) area of the rug.
5. A reflecting pool is in the shape of a circle with diametre of $20$ feet. What is the circumference of the pool?	6. A turntable is a circle with diametre of $10$ inches. What is the circumference of the turntable?
7. A circular saw has a diametre of $12$ inches. What is the circumference of the saw?	8. A round coin has a diametre of $3$ centimetres. What is the circumference of the coin?
9. A barbecue grill is a circle with a diametre of $2.2$ feet. What is the circumference of the grill?	10. The top of a pie tin is a circle with a diametre of $9.5$ inches. What is the circumference of the top?
11. A circle has a circumference of $163.28$ inches. Find the diametre.	12. A circle has a circumference of $59.66$ feet. Find the diametre.
13. A circle has a circumference of $17.27$ metres. Find the diametre.	14. A circle has a circumference of $80.07$ centimetres. Find the diametre.

In the following exercises, find the radius of the circle with given circumference.

15. A circle has a circumference of $150.72$ feet.	16. A circle has a circumference of $251.2$ centimetres.
17. A circle has a circumference of $40.82$ miles.	18. A circle has a circumference of $78.5$ inches.

Find the Area of Irregular Figures

In the following exercises, find the area of the irregular figure. Round your answers to the nearest hundredth.

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In the following exercises, solve.

39. A city park covers one block plus parts of four more blocks, as shown. The block is a square with sides $250$ feet long, and the triangles are isosceles right triangles. Find the area of the park.

A square is shown with four triangles coming off each side.

40. A gift box will be made from a rectangular piece of cardboard measuring $12$ inches by $20$ inches, with squares cut out of the corners of the sides, as shown. The sides of the squares are $3$ inches. Find the area of the cardboard after the corners are cut out.

A rectangle is shown. Each corner has a gray shaded square. There are dotted lines drawn across the side of each square attached to the next square.

41. Perry needs to put in a new lawn. His lot is a rectangle with a length of $120$ feet and a width of $100$ feet. The house is rectangular and measures $50$ feet by $40$ feet. His driveway is rectangular and measures $20$ feet by $30$ feet, as shown. Find the area of Perry’s lawn.

A rectangular lot is shown. In it is a home shaped like a rectangle attached to a rectangular driveway.

42. Denise is planning to put a deck in her back yard. The deck will be a $\text{20-ft}$ by $\text{12-ft}$ rectangle with a semicircle of diametre $6$ feet, as shown below. Find the area of the deck.

A picture of a deck is shown. It is shaped like a rectangle with a semi-circle attached to the top on the left side.

Everyday Math

43. Area of a Tabletop Yuki bought a drop-leaf kitchen table. The rectangular part of the table is a $\text{1-ft}$ by $\text{3-ft}$ rectangle with a semicircle at each end, as shown. a) Find the area of the table with one leaf up. b) Find the area of the table with both leaves up.

An image of a table is shown. There is a rectangular portion attached to a semi-circular portion. There is another semi-circular leaf folded down on the other side of the rectangle.

44. Painting Leora wants to paint the nursery in her house. The nursery is an $\text{8-ft}$ by $\text{10-ft}$ rectangle, and the ceiling is $8$ feet tall. There is a $\text{3-ft}$ by $\text{6.5-ft}$ door on one wall, a $\text{3-ft}$ by $\text{6.5-ft}$ closet door on another wall, and one $\text{4-ft}$ by $\text{3.5-ft}$ window on the third wall. The fourth wall has no doors or windows. If she will only paint the four walls, and not the ceiling or doors, how many square feet will she need to paint?

Writing Exercises

45. Describe two different ways to find the area of this figure, and then show your work to make sure both ways give the same area.

A geometric shape is shown. It is a vertical rectangle attached to a horizontal rectangle. The width of the vertical rectangle is 3, the left side is labeled 6, the bottom is labeled 9, and the width of the horizontal rectangle is labeled 3. The top of the horizontal rectangle is labeled 6, and the distance from the top of that rectangle to the top of the other rectangle is labeled 3.

46. A circle has a diametre of $14$ feet. Find the area of the circle a) using $3.14$ for $\pi$ b) using $\frac{22}{7}$ for $\pi$ . c) Which calculation to do prefer? Why?

Answers

1. a) 43.96 in. b) 153.86 sq. in.	3. a) 53.38 ft b) 226.865 sq. ft	5. 62.8 ft
7. 37.68 in.	9. 6.908 ft	11. 52 in.
13. 5.5 m	15. 24 ft	17. 6.5 mi
19. 16 sq. units	21. 30 sq. units	23. 57.5 sq. units
25. 12 sq. units	27. 67.5 sq. units	29. 89 sq. units
31. 44.81 sq. units	33. 41.12 sq. units	35. 35.13 sq. units
37. 95.625 sq. units	39. 187,500 sq. ft	41. 9400 sq. ft
43. a) 6.5325 sq. ft b) 10.065 sq. ft	45. Answers will vary.

Attributions

This chapter has been adapted from “Solve Geometry Applications: Circles and Irregular Figures” in Prealgebra (OpenStax) by Lynn Marecek, MaryAnne Anthony-Smith, and Andrea Honeycutt Mathis, which is under a CC BY 4.0 Licence. Adapted by Izabela Mazur. See the Copyright page for more information.

License

Icon for the Creative Commons Attribution 4.0 International License

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Use the Properties of Circles

Find the Area of Irregular Figures

Key Concepts

Glossary

Practice Makes Perfect

Use the Properties of Circles

Find the Area of Irregular Figures

Everyday Math

Writing Exercises

Answers

Attributions

License

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