Solve Geometry Applications: Circles and Irregular Figures

Use the Properties of Circles

Do you remember the properties of circles from Decimals and Fractions Together? We’ll show them here again to refer to as we use them to solve applications.

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

Try it.

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

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 circle
Step 3. Name. Choose a variable to represent it.	Let c = circumference of the circle
Step 4. Translate. Write the appropriate formula Substitute	\(C=2\pi r\) \(C=2\pi (2.5)\)
Step 5. Solve the equation.	\(C\approx 2(3.14)(2.5)\) \(C\approx 15\text{ft}\)
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.

ⓑ 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	\(A=\pi {r}^{2}\) \(A=\pi {(2.5)}^{2}\)
Step 5. Solve the equation.	\(A\approx (3.14){(2.5)}^{2}\) \(A\approx 19.625\ \text{sq. ft}\)
Step 6. Check. Yes. If we draw a square around the circle, its sides would be 5 ft, as shown in part ⓐ. 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.

We usually see the formula for circumference in terms of the radius \(r\) of the circle:

\[C=2\pi r\]

Condensed: the full section is in OpenStax Prealgebra 2e.

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

Try it.

Find the area of the shaded region.

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.

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.

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.

Example

Try it.

Find the area of the shaded region.

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 \(8-5\).

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

The area of the figure is \(36.5\) square units.

Example

Try it.

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

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.

The rectangle has a length of \(105\) m and a width of \(68\) m. The semi-circles have a diameter of \(68\) m, so each has a radius of \(34\) m.

Condensed: the full section is in OpenStax Prealgebra 2e.

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: \(A=\pi {r}^{2}\)

Solve Geometry Applications: Circles and Irregular Figures

Use the Properties of Circles

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

Try it.

The lid of a paint bucket is a circle with radius \(7\) inches. Find the ⓐ circumference and ⓑ area of the lid.

Solution

1. ⓐ 43.96 in.
2. ⓑ 153.86 sq. in.

Try it.

An extra-large pizza is a circle with radius \(8\) inches. Find the ⓐ circumference and ⓑ area of the pizza.

Try it.

A farm sprinkler spreads water in a circle with radius of \(8.5\) feet. Find the ⓐ circumference and ⓑ area of the watered circle.

Solution

1. ⓐ 53.38 ft
2. ⓑ 226.865 sq. ft

Try it.

A circular rug has radius of \(3.5\) feet. Find the ⓐ circumference and ⓑ area of the rug.

Try it.

A reflecting pool is in the shape of a circle with diameter of \(20\) feet. What is the circumference of the pool?

Solution

62.8 ft

Try it.

A turntable is a circle with diameter of \(10\) inches. What is the circumference of the turntable?

Try it.

A circular saw has a diameter of \(12\) inches. What is the circumference of the saw?

Solution

37.68 in.

Try it.

A round coin has a diameter of \(3\) centimeters. What is the circumference of the coin?

Try it.

A barbecue grill is a circle with a diameter of \(2.2\) feet. What is the circumference of the grill?

Solution

6.908 ft

Try it.

The top of a pie tin is a circle with a diameter of \(9.5\) inches. What is the circumference of the top?

Try it.

A circle has a circumference of \(163.28\) inches. Find the diameter.

Solution

52 in.

Try it.

A circle has a circumference of \(59.66\) feet. Find the diameter.

Try it.

A circle has a circumference of \(17.27\) meters. Find the diameter.

Solution

5.5 m

Try it.

A circle has a circumference of \(80.07\) centimeters. Find the diameter.

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

Try it.

A circle has a circumference of \(150.72\) feet.

Solution

24 ft

Try it.

A circle has a circumference of \(251.2\) centimeters.

Try it.

A circle has a circumference of \(40.82\) miles.

Solution

6.5 mi

Try it.

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.

In the following exercises, solve.

Try it.

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.

Solution

187,500 sq. ft

Try it.

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.

Try it.

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.

Solution

9400 sq. ft

Try it.

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 diameter \(6\) feet, as shown below. Find the area of the deck.

Condensed: the full section is in OpenStax Prealgebra 2e.

Сега ти. Ниту еден калкулатор не го решава овој, но деловите од него се комуникантни.

⌨ Тип ✎ Нацртај

Твојот математски проблем

π Реши

Вежбај (40)

Try each one on paper first. Reveal the answer to check; verified ones can be opened in the solver for every step.

1. Evaluate \({x}^{2}\) when \(x=5.\)

   Потруди се да го направиш тоа. Совет

   Одкриј го одговорот

   \(25\)

2. Using \(3.14\) for \(\pi ,\) approximate the (a) circumference and (b) the area of a circle with radius \(8\) inches.

   Потруди се да го направиш тоа. Совет

   Одкриј го одговорот

   (a) \(50.24\text{in}.\); (b) \(200.96\text{sq}.\text{in}\).

3. Simplify \(\frac{22}{7}{(0.25)}^{2}\) and round to the nearest thousandth.

   Потруди се да го направиш тоа. Совет

   Одкриј го одговорот

   \(\text{0}\text{.196}\)

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

   Потруди се да го направиш тоа. Совет

   Одкриј го одговорот

   ⓐ 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	\(C=2\pi r\) \(C=2\pi (2.5)\)
   Step 5. Solve the equation.	\(C\approx 2(3.14)(2.5)\) \(C\approx 15\text{ft}\)
   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.

   ⓑ 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	\(A=\pi {r}^{2}\) \(A=\pi {(2.5)}^{2}\)
   Step 5. Solve the equation.	\(A\approx (3.14){(2.5)}^{2}\) \(A\approx 19.625\ \text{sq. ft}\)
   Step 6. Check. Yes. If we draw a square around the circle, its sides would be 5 ft, as shown in part ⓐ. 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.

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

   Потруди се да го направиш тоа. Совет

   Одкриј го одговорот

   1. ⓐ 31.4 in.
   2. ⓑ 78.5 sq. in.

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

   Потруди се да го направиш тоа. Совет

   Одкриј го одговорот

   1. ⓐ 28.26 ft
   2. ⓑ 63.585 sq. ft

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

   Потруди се да го направиш тоа. Совет

   Одкриј го одговорот

   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.	\(C=\pi d\) \(C=\pi (4)\)
   Step 5. Solve the equation, using 3.14 for \(\pi .\)	\(C\approx (3.14)(4)\) \(C\approx 12.56\ \text{feet}\)
   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 diameter of the table is 12.56 feet.

8. Find the circumference of a circular fire pit whose diameter is \(5.5\) feet.

   Потруди се да го направиш тоа. Совет

   Одкриј го одговорот

   17.27 ft

9. If the diameter of a circular trampoline is \(12\) feet, what is its circumference?

   Потруди се да го направиш тоа. Совет

   Одкриј го одговорот

   37.68 ft

10. Find the diameter of a circle with a circumference of \(47.1\) centimeters.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    Step 1. Read the problem. Draw the figure and label it with the given information.
    Step 2. Identify what you are looking for.	the diameter of the circle
    Step 3. Name. Choose a variable to represent it.	Let d = the diameter of the circle
    Step 4. Translate.
    Write the formula. Substitute, using 3.14 to approximate \(\pi\).
    Step 5. Solve.
    Step 6. Check: \(47.1\overset{?}{=}(3.14)(15)\) \(47.1=47.1✓\)
    Step 7. Answer the question.	The diameter of the circle is approximately 15 centimeters.

11. Find the diameter of a circle with circumference of \(94.2\) centimeters.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    30 cm

12. Find the diameter of a circle with circumference of \(345.4\) feet.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    110 ft

13. Find the area of the shaded region.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    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.

    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.

    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.

14. Find the area of each shaded region:

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    28 sq. units

15. Find the area of each shaded region:

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    110 sq. units

16. Find the area of the shaded region.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    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 \(8-5\).

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

    The area of the figure is \(36.5\) square units.

17. Find the area of each shaded region.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    36.5 sq. units

18. Find the area of each shaded region.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    70 sq. units

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

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    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.

    The rectangle has a length of \(105\) m and a width of \(68\) m. The semi-circles have a diameter of \(68\) m, so each has a radius of \(34\) m.

20. Find the area:

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    103.2 sq. units

21. Find the area:

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    38.24 sq. units

22. The lid of a paint bucket is a circle with radius \(7\) inches. Find the ⓐ circumference and ⓑ area of the lid.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    1. ⓐ 43.96 in.
    2. ⓑ 153.86 sq. in.

23. An extra-large pizza is a circle with radius \(8\) inches. Find the ⓐ circumference and ⓑ area of the pizza.

    Потруди се да го направиш тоа. Совет

24. A farm sprinkler spreads water in a circle with radius of \(8.5\) feet. Find the ⓐ circumference and ⓑ area of the watered circle.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    1. ⓐ 53.38 ft
    2. ⓑ 226.865 sq. ft

25. A circular rug has radius of \(3.5\) feet. Find the ⓐ circumference and ⓑ area of the rug.

    Потруди се да го направиш тоа. Совет

26. A reflecting pool is in the shape of a circle with diameter of \(20\) feet. What is the circumference of the pool?

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    62.8 ft

27. A turntable is a circle with diameter of \(10\) inches. What is the circumference of the turntable?

    Потруди се да го направиш тоа. Совет

28. A circular saw has a diameter of \(12\) inches. What is the circumference of the saw?

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    37.68 in.

29. A round coin has a diameter of \(3\) centimeters. What is the circumference of the coin?

    Потруди се да го направиш тоа. Совет

30. A barbecue grill is a circle with a diameter of \(2.2\) feet. What is the circumference of the grill?

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    6.908 ft

31. The top of a pie tin is a circle with a diameter of \(9.5\) inches. What is the circumference of the top?

    Потруди се да го направиш тоа. Совет

32. A circle has a circumference of \(163.28\) inches. Find the diameter.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    52 in.

33. A circle has a circumference of \(59.66\) feet. Find the diameter.

    Потруди се да го направиш тоа. Совет

34. A circle has a circumference of \(17.27\) meters. Find the diameter.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    5.5 m

35. A circle has a circumference of \(80.07\) centimeters. Find the diameter.

    Потруди се да го направиш тоа. Совет

36. A circle has a circumference of \(150.72\) feet.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    24 ft

37. A circle has a circumference of \(251.2\) centimeters.

    Потруди се да го направиш тоа. Совет

38. A circle has a circumference of \(40.82\) miles.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    6.5 mi

39. A circle has a circumference of \(78.5\) inches.

    Потруди се да го направиш тоа. Совет

40. 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.

    Потруди се да го направиш тоа. Совет

    Одкриј го одговорот

    187,500 sq. ft

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Симболи кои се користат тука

Допрете било кој симбол за целосната дефиниција, слика, и што значи секоја буква во неа.

\pipi
Ratio of a circle's circumference to its diameter, 3.14159…\approxapproximately equal
Equal to the precision shown, not exactly.\thetatheta
The usual name for an angle.^\circdegrees
1/360 of a full turn. 180° = π radians.\sinsine, cosine, tangent
Ratios of sides in a right triangle; coordinates on the unit circle.\angle ABCangle, triangle
The angle at B between BA and BC; the triangle with those vertices.\parallelparallel, perpendicular, congruent, similar
Never meet; meet at 90°; identical shape and size; same shape.

Како да го направиш тоа?: Solve Geometry Applications: Circles and Irregular Figures

1. Use the properties of circles
2. Find the area of irregular figures

Прашања што луѓето ги поставуваат

Why does every triangle have angles adding to 180°?

Draw a line through one vertex parallel to the opposite side: the two alternate angles equal the other two angles of the triangle, and the three angles at the vertex lie on a straight line. That is Euclid's proof, and it is why the fact holds only on a flat plane.

When do I use the law of sines versus the law of cosines?

Cosines when you know three sides, or two sides and the angle between them. Sines when you know an angle and the side opposite it, plus one more piece.

What is the difference between area and perimeter?

Perimeter is the length of the boundary (one dimension, measured in metres); area is the amount of surface inside (two dimensions, square metres). Doubling every side doubles the perimeter but quadruples the area.

Деловите од оваа страница се прилагодени од OpenStax Prealgebra 2e (CC BY-NC-SA 4.0). Кондензација и повторно објаснување тука; грешките се наши.

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