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Circle math

Area of a circle Circumference of a circleArc of a circle

Trigonometry

Law of sinesLaw of cosines

Rounding Square rootsHere you will learn about sectors of a circle, including how to find the area of a sector, the perimeter of a sector, and solve problems involving sectors of circles.

Students will first learn about the sector of a circle as part of geometry in high school.

A **sector of a circle** is formed when a circle is divided using two radii.

Each sector has an angle between the two **radii**. The sector with an angle less than 180 degrees is called a **minor sector** and the sector with an angle greater than 180 degrees is called a **major sector**. If the **central angle** formed equals 180 degrees, the two sectors would be **semicircles**.

For example,

You will need to know how to solve problems involving the area of the sector of a circle and the perimeter of a sector of a circle. You may be asked to give exact answers in terms of \pi or using decimals.

Prepare for math tests in your state with these Grade 3 to Grade 6 practice assessments for Common Core and state equivalents. 40 multiple choice questions and detailed answers to support test prep, created by US math experts covering a range of topics!

DOWNLOAD FREEPrepare for math tests in your state with these Grade 3 to Grade 6 practice assessments for Common Core and state equivalents. 40 multiple choice questions and detailed answers to support test prep, created by US math experts covering a range of topics!

DOWNLOAD FREEThe **area of a sector** is the space inside the section of the circle created by two radii and an arc. It is a fraction of the area of the full circle.

The formula to find the area of a sector is,

\text{Area of a sector}=\cfrac{\theta}{360}\times\pi{r^2}

\theta = angle of the sector

r = radius of the circle

The **perimeter of a sector** is the distance around a sector.

You can calculate the perimeter of a sector by adding together the lengths of the two radii and the **arc length** of the sector.

To find the length of an arc of a sector you need to use the formula,

\text{Arc length}=\cfrac{\theta}{360}\times\pi\times{d}

\theta = \text{ Angle of the sector}

r = Radius of the circle

or

\text{Arc length}=\cfrac{\theta}{360}\times{2\pi{r}}

\theta = Angle of the sector

r = Radius of the circle

The total perimeter of a sector formula would be,

\text{Perimeter of a sector } = \text{ Arc length } + \text{ radius } + \text{ radius} \begin{aligned} &=\cfrac{\theta}{360}\times\pi{d}+d \\\\ &\text{or} \\\\ &=\cfrac{\theta}{360}\times{2\pi{r}} +2r \end{aligned}How does this relate to high school math?

**High School – Geometry – Circles (HS.G.C.B.5)**Derive using similarity the fact that the length of the arc intercepted by an angle is proportional to the radius, and define the radian measure of the angle as the constant of proportionality; derive the formula for the area of a sector.

In order to calculate the area of a sector:

**Find the length of the radius.****Find the size of the angle creating the sector.****Substitute the values you know into the formula for the area of a sector.****Calculate the answer.**

Calculate the area of the sector shown below. Give your answer to the nearest tenth.

**Find the length of the radius.**

Radius r=8\mathrm{~cm}

2**Find the size of the angle creating the sector.**

3**Substitute the values you know into the formula for the area of a sector.**

4**Calculate the answer.**

The question asked you to round your answer to the nearest tenth.

\begin{aligned}\text{Area of a sector}&=\cfrac{184}{9}\pi\mathrm{~cm^{2}} \\\\ &=64.228…\mathrm{~cm^2} \\\\ &=64.2\mathrm{~cm^{2}}\text{ (1dp)} \end{aligned}Calculate the area of the sector shown below. Give your answer to the nearest tenth.

**Find the length of the radius.**

Radius r=4\mathrm{~m}

**Find the size of the angle creating the sector.**

The angle creating the sector is the angle included between the two radii. To find it in this case, you need to use the angle fact: angles around a point add to 360^{\circ}.

360-240=120^{\circ}

The angle creating the sector is 120^{\circ}.

**Substitute the values you know into the formula for the area of a sector.**

\begin{aligned}\text{Area of a sector}&=\cfrac{\theta}{360}\times\pi\times{r^2} \\\\
&=\cfrac{120}{360}\times\pi\times{4^2} \end{aligned}

**Calculate the answer.**

\begin{aligned}&=16.75516083 \\\\
&=16.8\mathrm{~m}^{2} \end{aligned}

Below is a sector of a circle. The diameter of the circle is 12\mathrm{~cm}. Calculate the area of the sector. Give your answer to the nearest tenth.

**Find the length of the radius.**

Diameter d=12\mathrm{~cm}

The radius is half of the diameter, so the radius r=6\mathrm{~cm}.

**Find the size of the angle creating the sector.**

The angle creating the sector is the angle included between the two radii. To find it in this case, you need to use the angle fact: angles around a point add to 360^{\circ}.

360-205=155^{\circ}

The angle creating the sector is 155^{\circ}.

**Substitute the values you know into the formula for the area of a sector.**

\begin{aligned}\text{Area of a sector}&=\cfrac{\theta}{360}\times\pi\times{r^2} \\\\
&=\cfrac{155}{360}\times\pi\times{6^2} \end{aligned}

**Calculate the answer.**

\begin{aligned}&=48.69468613 \\\\
&=48.7\mathrm{~cm}^{2} \end{aligned}

In order to calculate the perimeter of a sector:

**Find the length of the radius.****Find the size of the angle creating the sector.****Substitute the values you know into the formula for the arc length.****Add together the arc length and the two radii.**

Calculate the perimeter of the sector shown below. Give your answer to the nearest tenth.

**Find the length of the radius.**

Radius r=5.5\mathrm{~cm}

**Find the size of the angle creating the sector.**

Angle \theta=117^{\circ}

**Find the arc length of the sector.**

\begin{aligned}\text{Arc length}&=\cfrac{\theta}{360}\times{2}\times\pi\times{r} \\\\ &=\cfrac{117}{360}\times{2}\times\pi\times{5.5} \\\\ &=\cfrac{143}{40}\pi \end{aligned}

**Add together the arc length and the two radii.**

Arc length: \cfrac{143}{40}\pi\mathrm{~cm}

Radius: 5.5\mathrm{~cm}

Calculate the perimeter of the sector shown below. Give your answer to the nearest tenth.

**Find the length of the radius.**

Radius r=4\mathrm{~m}

**Find the size of the angle creating the sector.**

The angle creating the sector is the angle included between the two radii. To find it in this case, you need to use the angle fact: angles around a point add to 360^{\circ}.

360-240=120^{\circ}

The angle creating the sector is 120^{\circ}.

**Substitute the values you know into the formula for the arc length.**

\begin{aligned}\text{Arc length}&=\cfrac{\theta}{360}\times{2}\times\pi\times{r} \\\\
&=\cfrac{120}{360}\times{2}\times\pi\times{4} \\\\
&=8.37758041\mathrm{~cm} \end{aligned}

**Add together the arc length and the two radii.**

\begin{aligned}&8.377580+4+4 \\\\
&=16.37758041 \\\\
&=16.4\mathrm{~cm} \end{aligned}

Below is a sector of a circle. The diameter of the circle is 12\mathrm{~cm}. Calculate the perimeter of the sector. Give your answer to the nearest tenth.

**Find the length of the radius.**

Diameter d=12\mathrm{~cm}

The radius is half of the diameter, so, the radius r=6\mathrm{~cm}.

You can use the radius or diameter as long as you use the correct formula for each.

**Find the size of the angle creating the sector.**

360-205=155^{\circ}

The angle creating the sector is 155^{\circ}.

**Substitute the values you know into the formula for the arc length.**

\begin{aligned}\text{Arc length}&=\cfrac{\theta}{360}\times{2}\times\pi\times{r} \\\\
&=\cfrac{155}{360}\times{2}\times\pi\times{6} \\\\
&=16.23156204\mathrm{~cm} \end{aligned}

**Add together the arc length and the two radii.**

\begin{aligned}&16.23156204+6+6 \\\\ &=28.23156204 \\\\ &=28.2\mathrm{~cm} \end{aligned}

- Have students work in groups to complete worksheets involving sectors, encouraging peer teaching and discussion.

- Provide practice questions involving real-life objects like pizza slices, pie charts, and sections of a Ferris wheel.

- Create study materials with key terms like “sector,” “central angle,” “radius,” “arc,” and “circle.”

**Confusing the segment/sector**

A segment of a circle is made from a chord while a sector will have lines (radii) coming from the origin. Sectors are a part of a circle – it can be helpful to think of a sector as a pizza slice.

**Finding the area of the whole circle not the area of the sector**

Remember to find the fraction of the circle that makes the sector not just the area of the whole circle. Use \cfrac{\theta}{360}\times\pi{r^2} not \pi{r^2} to find the area of a sector.

**Finding the length of the arc not the perimeter of the sector**

You must remember that the perimeter of the sector is the combined length of the arc and the two radii.

**Using incorrect units**

Remember the perimeter is a length and therefore the units will not be squared or cubed. Square units are for area and cubed units are for volume.

**Incorrect angle unit set on calculator**

All angles are measured in degrees. Make sure that your calculator has a small ‘d’ for degrees at the top of the screen rather than an ‘r’ for radians.

- Angles of a circle
- Segment of a circle
- Area of a segment of a circle
- Arc length formula
- Perimeter of a sector
- Area of a sector

1. Find the area of the sector in terms of \pi.

16\pi\mathrm{~cm}^2

8\pi\mathrm{~cm}^2

\cfrac{8}{3} \, \pi\mathrm{~cm}^2

8.4\pi\mathrm{~cm}^2

Substitute the values you know into the formula for the area of a sector.

\begin{aligned}&\text{Area of a sector}=\cfrac{\theta}{360}\times\pi\times{r^2} \\\\ &=\cfrac{60}{360}\times\pi\times{4^2} \\\\ &=\cfrac{8}{3} \, \pi\mathrm{~cm}^{2} \end{aligned}

2. A sector has an area of 18\pi\mathrm{~cm}^2 and a radius of 12\mathrm{~cm}. What is the angle of the sector?

45^{\circ}

30^{\circ}

60^{\circ}

90^{\circ}

Substituting the known values into the formula for the area of a sector,

\cfrac{\theta}{360}\times\pi\times{12^2}=18\pi

Solving for angle \theta,

\theta=45^{\circ}

3. A sector has an area of 21\pi\mathrm{~cm^2} and an angle of 210^{\circ}. Find the radius of the sector.

12\mathrm{~cm}

6\mathrm{~cm}

9\mathrm{~cm}

36\mathrm{~cm}

Substituting the known values into the formula for the area of a sector,

\cfrac{210}{360}\times\pi\times{r^2}=21\pi

Solving for radius r,

\begin{aligned}r^{2}&=36 \\\\\ r&=\sqrt{36}=6\mathrm{~cm} \end{aligned}

4. Find the perimeter of the sector to the nearest hundredth.

8.19\mathrm{~cm}

16.38\mathrm{~cm}

10.09\mathrm{~cm}

12.19\mathrm{~cm}

Substituting the known values into the formula for the arc length,

\begin{aligned}\text{Arc length}&=\cfrac{\theta}{360}\times{2}\times\pi\times{r} \\\\ &=\cfrac{60}{360}\times{2}\times\pi\times{4} \\\\ &=4.188790205\mathrm{~cm} \end{aligned}

Adding together the arc length and the two radii,

\begin{aligned}&4.188790205+4+4 \\\\ &=12.188790205 \\\\ &=12.19\mathrm{~cm} \end{aligned}

5. A sector has a radius 5\mathrm{~cm} and a perimeter 2\pi+10\mathrm{~cm}. What was the angle of the sector?

72^{\circ}

60^{\circ}

90^{\circ}

108^{\circ}

Substituting the known values into the formula for the total perimeter,

\left(\cfrac{\theta}{360}\times{2}\times\pi\times{5}\right)+5+5=2\pi+10

Solving for \theta,

\theta=72^{\circ}

6. A sector has an angle 24^{\circ} and a perimeter 29.03\mathrm{~cm}. What was the radius of the sector to the nearest centimeter?

24\mathrm{~cm}

12\mathrm{~cm}

9\mathrm{~cm}

10\mathrm{~cm}

Substituting the known values into the formula for the total perimeter,

\left(\cfrac{24}{360}\times{2}\times\pi\times{r}\right)+2r=29.03

Rearranging to change the subject of the formula to r,

\begin{aligned}r\left(\cfrac{48\pi}{360}+2\right)&=29.03 \\\\ &=29.03\div\left(\cfrac{48\pi}{360}+2\right) \\\\ &=12.00142701 \\\\ &=12\mathrm{~cm} \end{aligned}

A sector of a circle is a part of the circle enclosed by two radii and the arc between them, with the endpoints of the arc lying on the circumference of the circle.

Here are the key formulas related to the sector of a circle with all angles in degrees:

\text{ Arc of the sector } =\cfrac{\theta}{360}\times2\pi{r}

\text{ Area } =\cfrac{\theta}{360}\times\pi{r^2}

\text{ Perimeter } =\cfrac{\theta}{360}\times2\pi{r}+2r

Understanding sectors is important for solving problems in geometry, trigonometry, and real-world applications such as engineering, architecture, and various fields that involve circular shapes and motions.

The central angle of a semicircle is 180 degrees or \pi radians.

- Circle theorems
- Prism shape
- Types of data

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[FREE] Common Core Practice Tests (Grades 3 to 6)

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Prepare for math tests in your state with these Grade 3 to Grade 6 practice assessments for Common Core and state equivalents.

40 multiple choice questions and detailed answers to support test prep, created by US math experts covering a range of topics!