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Parts of a circle

Irrational numbers

Rounding decimals

Rounding to significant figuresThis topic is relevant for:

Here we will learn about calculating the area of a circle including how to calculate the area of a circle given the radius, how to calculate the area of a circle given the diameter and how to calculate the area of a circle given the circumference.

There are also area of a circle worksheets based on Edexcel, AQA and OCR exam questions, along with further guidance on where to go next if you’re still stuck.

The **area of a circle** is given by the area of a circle formula which is made by using a specific relationship between the radius of a circle and its area.

**Area of a circle formula:**

we usually simplify this to

E.g.

What is the area of a circle with radius 3cm ?

\pi (pronounced pi) represents the ratio of the circumference of a circle to its diameter.

For all circles if you divide the length of the circumference by the length of the diameter you get the value \pi .

**Note**: \pi is an **irrational **number which means it cannot be written as a fraction. It is a non recurring decimal and has an approximate value of 3.14159…

In GCSE you should use the \pi button on your Casio calculator when working with \pi .

To get this you need to press [SHIFT][ \times 10^x ].

Alternatively you can use the value 3.141 instead.

Sometimes a question may ask you to leave an answer in terms of pi

E.g

6 \times \pi = 6\pi (this is an answer in terms of pi)

8 \times \pi = 8\pi (this is an answer in terms of pi)

10 \times \pi = 31.41... (this is an answer not in terms of pi)

In order to calculate the area of a circle:

**Find the radius of the circle.****Use the formula \text{Area of a circle} = \pi r^2 to calculate the area of the circle.****Give your answer clearly with the correct units.**

Get your free area of a circle worksheet of 20+ questions and answers. Includes reasoning and applied questions.

COMING SOONGet your free area of a circle worksheet of 20+ questions and answers. Includes reasoning and applied questions.

COMING SOONA circle has a radius of 6cm , calculate its area.

Give your answer to 2dp .

**Find the radius of the circle.**

The radius is given in the question

Radius =6cm

2**Use the formula \pi r^2 to calculate the area of the circle.**

3**Give your answer clearly with the correct units.**

*Remember the question asks you to round your answer to ‘2 decimal places’*

A circle has a diameter of 10mm. , calculate its area.

Give your answer to 1dp .

**Find the radius of the circle.**

In this question the question gives you the diameter. You need the radius to find the area of the circle.

Remember the diameter of the circle is twice the radius.

Diameter = 10mm

Radius =10mm \div 2

Radius = 5mm

**Use the formula \pi r^2 to calculate the area of the circle.**

\begin{aligned}
&\pi r^2\\\\
&= \pi \times r\times r\\\\
&= \pi \times5\times 5\\\\
&= 25\pi \\\\
&=78.53981...
\end{aligned}

**Give your answer clearly with the correct units.**

*Remember the question asks you to round your answer to ‘1 decimal place’*

78.5mm^2

A circle has a radius of 8m. , calculate its area.

Give your answer in terms of \pi .

**Find the radius of the circle.**

The radius is given in the question

Radius = 8m

**Use the formula \pi r^2 to calculate the area of the circle.**

\begin{aligned} &\pi r^2\\\\ &= \pi \times r\times r\\\\ &= \pi \times8\times 8\\\\ &= 64\pi \end{aligned}

**Give your answer clearly with the correct units.**

*Remember the question asks you to give your answer to ‘in terms of* \pi .

A circle has a diameter of 420km. , calculate its area.

Give your answer in terms of \pi

**Find the radius of the circle.**

In this question the question gives you the diameter. You need the radius to find the area of the circle.

Diameter = 420km

Radius = 420km \div 2

Radius = 210km

**Use the formula \pi r^2 to calculate the area of the circle.**

\begin{aligned} &\pi r^2\\\\ &= \pi \times r\times r\\\\ &= \pi \times210\times 210\\\\ &= 44100\pi \end{aligned}

**Give your answer clearly with the correct units.**

*You leave the answer in the form* 44100 \; \pi

A circle has a circumference of 21cm. , calculate its area.

Give your answer to 2dp .

**Find the radius of the circle.**

The question gives you the circumference of the circle. But you need the radius

You know that the circumference of a circle is equal to 2\pi r

This means you can find the radius of the circle from the circumference, see below:

Circumference = 2\pi r

Circumference = 21

21 = 2\pi r

Divide both sides by 2\pi

\frac{21}{2\pi}= r

Notice how we leave our answer in terms of \pi
at this stage. This is so you do not cause a rounding error later on in the question.

**Use the formula \pi r^2 to calculate the area of the circle.**

\begin{aligned}
&\pi r^2\\\\
&= \pi \times r\times r\\\\
&= \pi \times\frac{21}{2\pi}\times \frac{21}{2\pi}\\\\
&=35.09366..
\end{aligned}

**Give your answer clearly with the correct units.**

*Remember the question asks you to round your answer to ‘2 decimal place’*

A semicircle has a diameter of 20m. , calculate its area.

Give your answer in terms of \pi

**Find the radius of the circle.**

In this question the question gives you the diameter. You need the radius to find the area of the circle.

Remember the diameter of the circle is twice the radius.

Diameter = 20m

Radius = 20m \div 2

Radius = 10m

**Use the formula \pi r^2 to calculate the area of the circle.**

\begin{aligned} &\pi r^2\\\\ &= \pi \times r\times r\\\\ &= \pi \times 10\times 10\\\\ &= 100\pi \end{aligned}

This represents the area of a whole circle with a diameter of 20m.

A semi circle has half the area of a full circle so you need to divide your answer by two. Remember to keep it in terms of \pi .

\begin{aligned}
&100\pi \div2 \\\\
&50\pi
\end{aligned}

**Give your answer clearly with the correct units.**

50 \pi m^2

**Radius**

You must have the radius to find the area of a circle by using the formula. If a question does not give the radius directly then it must be calculated.

**Correct units**

When working with area you must always give the correct units squared

E.g.

cm^2 , m^2, km^2 etc.

**Rounding**

It is important to only round at the end of the question and ensure you are round to what the question specifies.

E.g. to

**I**\pi**n terms of**

Sometimes the question may ask you to give the answer in terms of \pi ’. This means you do not give the numerical answer that is produced when you multiply it by \pi

E.g.

6 x \pi = 6 \pi (this is an answer in terms of pi)

6 x \pi = 18.8495592… (this answer is not in terms of pi)

**Misuse of calculator**

Ensure you know how to correctly use the \pi button on your calculator.

1. A circle has a diameter of 6cm . What is the radius of the circle?

6cm

3cm

12cm

6\pi cm

The diameter of the circle is twice the size of the radius. Therefore to find the radius you can divide the diameter by 2.

6cm \div 2 = 3cm

2. Which of these answers is in terms of \pi ?

31.4cm

20m

10\pi cm

24.5

10 \pi means 10 lots of \pi

3. A circle has a radius of 1cm. What is its area to 1 decimal place?

\pi cm^2

\pi cm

3.1cm

3.1 cm^2

Area of a circle = \pi r^2

\pi \times 1 \times 1 is equal to 3.1415…

This answer is correctly rounded to 1 decimal place and has the correct units.

4. A circle has a radius of 1cm. What is its area in terms of \pi ?

\pi cm^2

\pi cm

3.1cm

3.1 cm^2

Area of a circle = \pi r^2

\pi \times 1 \times 1 is equal to \pi

This answer is correctly given in terms of \pi and has the correct units.

5. A circle has a diameter of 2cm. What is its area in terms of \pi ?

4\pi cm^2

\pi cm^2

3.1cm

3.1 cm^2

You must first divide the diameter by 2 to find the radius, 2cm divided by 2 is equal to 1cm.

Therefore the radius is 1cm

Area of a circle = \pi r^2

\pi \times 1 \times 1 is equal to \pi

This answer is correctly given in terms of \pi and has the correct units.

6. A circle has a diameter of 100cm. What is its area to the nearest whole number?

31416 cm^2

31415 cm^2

31415.9 cm^2

10000\pi cm^2

Area of a circle = \pi r^2

\pi \times 100 \times 100 is equal to 31415.92654…

Which is 31416 cm^2 rounded to the nearest whole number.

1. The radius of a circle is 3.5 cm .

Work out the area of the circle.

Give your answer correct to 3 significant figures.

**(3 marks)**

Show answer

\pi \times3.5\times 3.5 or 38.4845… seen

**(1)**

38.5

**(1)**

cm^2

**(1)**

2. The radius of a circle is 17.2 m .

Work out the area of the circle.

Give your answer correct to 2 decimal places.

**(3 marks)**

Show answer

\pi \times17.2\times 17.2 or 929.408… seen

**(1)**

929.41

**(1)**

m^2

**(1)**

3. The diameter of a circle is 20mm .

Work out the area of the circle.

Give your answer in terms of \pi .

**(3 marks)**

Show answer

\pi \times10\times 10

**(1)**

100\pi

**(1)**

mm^2

**(1)**

4. The diameter of a circle is 15cm .

Work out the area of the circle.

Give your answer to 1 decimal place.

**(3 marks)**

Show answer

\pi \times7.5\times 7.5

**(1)**

176.7 (1.d.p)

**(1)**

cm^2

**(1)**

5. The circumference of a circle is 72cm .

Work out the area of the circle.

Give your answer correct to the nearest integer.

**(4 marks)**

Show answer

\frac{72}{2\pi}= r or \frac{36}{\pi}= r seen

**(1)**

\pi \times\frac{72}{2\pi}\times \frac{72}{2\pi}

**(1)**

412.52961… seen

**(1)**

413

**(1)**

6. A tile is in the shape of a semicircle.

The perimeter of a semi circle is 12.85cm and the length of the arc is 7.85cm .

Work out the total area of the tile.

Give your answer correct to the nearest integer.

**(4 marks)**

Show answer

Length of diameter = 5cm

**(1)**

6.25\pi or 19.634… *seen*

**(1)**

“19.634…” ÷ 2 or 9.817… *seen*

**(1)**

10

**(1)**

7. The area of a circle is 64\pi cm^2.

Calculate the radius of the circle.

**(3 marks)**

Show answer

64\pi = \pi r^2

**(1)**

64 = r^2

**(1)**

r=8

**(1)**

*Only positive value should be given*

You have now learned how to:

- Identify and apply circle definitions and properties, including: centre, radius, chord, diameter, circumference
- Know the formulae for area of a circle
- Give answers in terms of \pi
- Calculate area of 2D shapes including circles and semi-circles

- Circumference of a circle
- Arc lengths
- Area of a sector
- Perimeter of a sector
- Circle graph
- Equation of a circle
- Circle theorems
- Surface area and volume of spheres

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#### FREE GCSE Maths Practice Papers - 2022 Topics

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Practice paper packs based on the advanced information for the Summer 2022 exam series from Edexcel, AQA and OCR.

Designed to help your GCSE students revise some of the topics that will come up in the Summer exams.