Ready to master areas of circles and sectors? This guide... Show more
Geometry512 views·Updated May 22, 2026·2 pagesExploring Areas of Circles and Circular Sectors
Josephine McCotter@jp.mccotter
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Area of Circles and Semicircles
Ever wondered how much space a circle takes up? The area of a circle equals π × r². For example, with a circle of radius 2.9 ft, we calculate π × 2.9² ≈ 26.42 square feet. When you have the diameter instead (like 35 mm), simply divide by 2 first to get the radius (17.5 mm).
Semicircles are just half of a circle, so their area is (π × r²) ÷ 2. For a semicircle with radius 9.4 inches, we get (π × 9.4²) ÷ 2 ≈ 138.80 square inches. For a semicircle with radius 22 inches, the area is about 760.27 square inches.
Sometimes you'll need to work backward. If you know the area of a circle is 490.87 square feet, you can find the radius by using r = √(A ÷ π). This gives a radius of 12.5 feet, making the diameter 25 feet. Similarly, for a semicircle with area 76.97 km², you'd use r = √(2A ÷ π) to get approximately 7 km.
Pro Tip: Always check if you're working with the radius or diameter! Mixing these up is a common mistake that leads to answers that are off by a factor of 2 or 4.
When finding the area of a sector (a "pizza slice" of a circle), use the formula (θ ÷ 360°) × π × r², where θ is the central angle. For example, a sector with radius 11 inches and angle 64° has an area of (64 ÷ 360) × π × 11² ≈ 67.58 square inches.
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Area of Circle Sectors
Sectors are like pizza slices cut from a circle. To find their area, you need to know the central angle (θ) and radius (r). The formula is (θ ÷ 360°) × π × r², which gives you the fraction of the circle's area based on the angle.
When working with sectors, you'll often need to do some angle calculations first. For example, if a circle shows angles of 136° + 136° = 272°, then your sector takes up (272 ÷ 360) × π × 9² ≈ 192.27 square centimeters. For trickier problems, subtract known angles from 360° to find the sector angle.
Some problems may require you to identify the radius correctly. In problem #13, BD is the diameter (18 cm), so the radius is 9 cm. In problem #16, SR represents the diameter (26 m), making the radius 13 m. Always check whether you're given the radius or diameter!
Remember: When calculating sector areas, the angle must be expressed as a fraction of the full circle (360°). So a 90° angle represents 1/4 of the circle's area.
Sometimes you'll need to piece together angles from multiple sections. In problem #14, you need to add angles (108° + 108° = 216°), then subtract from 360° to get 144° for the desired sector. Using (144 ÷ 360) × π × 5.3² gives approximately 35.30 square kilometers.
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Geometry512 views·Updated May 22, 2026·2 pagesExploring Areas of Circles and Circular Sectors
Josephine McCotter@jp.mccotter
Ready to master areas of circles and sectors? This guide breaks down how to calculate these shapes with step-by-step solutions. You'll learn to find areas using formulas, handle different angles, and solve for missing dimensions.
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Area of Circles and Semicircles
Ever wondered how much space a circle takes up? The area of a circle equals π × r². For example, with a circle of radius 2.9 ft, we calculate π × 2.9² ≈ 26.42 square feet. When you have the diameter instead (like 35 mm), simply divide by 2 first to get the radius (17.5 mm).
Semicircles are just half of a circle, so their area is (π × r²) ÷ 2. For a semicircle with radius 9.4 inches, we get (π × 9.4²) ÷ 2 ≈ 138.80 square inches. For a semicircle with radius 22 inches, the area is about 760.27 square inches.
Sometimes you'll need to work backward. If you know the area of a circle is 490.87 square feet, you can find the radius by using r = √(A ÷ π). This gives a radius of 12.5 feet, making the diameter 25 feet. Similarly, for a semicircle with area 76.97 km², you'd use r = √(2A ÷ π) to get approximately 7 km.
Pro Tip: Always check if you're working with the radius or diameter! Mixing these up is a common mistake that leads to answers that are off by a factor of 2 or 4.
When finding the area of a sector (a "pizza slice" of a circle), use the formula (θ ÷ 360°) × π × r², where θ is the central angle. For example, a sector with radius 11 inches and angle 64° has an area of (64 ÷ 360) × π × 11² ≈ 67.58 square inches.
2
of 2Sign up to see the content. It's free!
- Access to all documents
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Area of Circle Sectors
Sectors are like pizza slices cut from a circle. To find their area, you need to know the central angle (θ) and radius (r). The formula is (θ ÷ 360°) × π × r², which gives you the fraction of the circle's area based on the angle.
When working with sectors, you'll often need to do some angle calculations first. For example, if a circle shows angles of 136° + 136° = 272°, then your sector takes up (272 ÷ 360) × π × 9² ≈ 192.27 square centimeters. For trickier problems, subtract known angles from 360° to find the sector angle.
Some problems may require you to identify the radius correctly. In problem #13, BD is the diameter (18 cm), so the radius is 9 cm. In problem #16, SR represents the diameter (26 m), making the radius 13 m. Always check whether you're given the radius or diameter!
Remember: When calculating sector areas, the angle must be expressed as a fraction of the full circle (360°). So a 90° angle represents 1/4 of the circle's area.
Sometimes you'll need to piece together angles from multiple sections. In problem #14, you need to add angles (108° + 108° = 216°), then subtract from 360° to get 144° for the desired sector. Using (144 ÷ 360) × π × 5.3² gives approximately 35.30 square kilometers.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
Where can I download the Knowunity app?
You can download the app in the Google Play Store and in the Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content in Geometry
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan SiOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha KlichAndroid user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
AnnaiOS user