Area and Circumference of a Circle
 Area and Circumference of a Circle – Definition
 Steps to Find the Area of a Circle
 Steps to Find the Circumference of a Circle
 Area and Circumference of a Circle – Exercises
 Area and Circumference of a Circle – RealWorld Application
 Area and Circumference of a Circle – Summary
 Area and Circumference of a Circle – Frequently Asked Questions
Learning text on the topic Area and Circumference of a Circle
Area and Circumference of a Circle – Definition
Circles are everywhere in our daily lives, from the wheels on a bus to the pizzas we enjoy. Understanding the area and circumference of a circle is not just a fundamental aspect of geometry, but also a practical skill used in various realworld scenarios.
The area of a circle measures the space inside it, while the circumference is the distance around the circle. These are calculated using the circle's radius or diameter and the constant Pi ($\pi$).
The key to working with circles is knowing their radius (the distance from the centre to the edge) and diameter (the distance across the circle, passing through the centre). The diameter is always twice the radius.
Concept  Measurement Description  Formula 

Area of a Circle  Measures the space inside the circle  $A = \pi r^2$ 
Circumference of a Circle  Measures the distance around the circle  $C = 2\pi r$ or $C = \pi d$ 
Steps to Find the Area of a Circle
Find the area of a circle with a radius of $4$ cm.
Identify the radius: The radius is $4$ cm.
Use the area formula: The formula for the area of a circle is $A = \pi r^2$.
Substitute in the value for the radius: Substitute the radius into the formula: $A = \pi \times 4^2$.
Calculate: Square the radius ($4$ cm) to get $16$ and then multiply by $\pi$.
Result: The area of the circle is $16\pi$ cm², or approximately $50.27$ cm² to 2 decimal places.
Steps to Find the Circumference of a Circle
Find the circumference of a circle with a radius of $4$ cm.
Identify the radius: The radius of our circle is $4$ cm.
Use the circumference formula: The formula for the circumference of a circle is $C = 2\pi r$.
Substitute in the value for the radius: Substitute the radius into the formula: $C = 2\pi \times 4$.
Calculate: Multiply $2$ by $\pi$ and then by $4$ to get the circumference.
Result: The circumference of the circle is $8\pi$ cm, or approximately $25.13$ cm to 2 decimal places.
Converting Between Radius and Diameter
 To find the diameter if you know the radius, multiply the radius by $2$.
 To find the radius if you know the diameter, divide the diameter by $2$.
Area and Circumference of a Circle – Exercises
Practise finding the area and circumference of circles!
Area and Circumference of a Circle – RealWorld Application
Knowing the area and circumference of a circle is practical for everyday tasks. Use the area for covering spaces like painting circles or gardening. Circumference helps when measuring around objects, like fencing a garden or sizing a table edge.
Area and Circumference of a Circle – Summary
Key Learnings from this Text:
 The area of a circle is found with $A = \pi r^2$.
 The circumference is calculated as $C = 2\pi r$ or $C = \pi d$.
 Understanding these concepts is crucial for realworld applications like planning and construction.
 Remember to double the radius to get the diameter, and halve the diameter to find the radius.
For more on area, have a look at area of right angled triangles.
Area and Circumference of a Circle – Frequently Asked Questions
Area and Circumference of a Circle exercise

Demonstrate your understanding of the area of a circle.
HintsThe radius or $r$ of this circle is $7$ cm.
The diameter or $d$ of this circle is $14$ cm.
The radius is half of the diameter.
As long as the $r$ (radius) is known, it is possible to find the area of a circle. The radius can be substituted in the formula for $r$.
$r=7$ cm
$A=\pi r^2$
$A=\pi (7^2)$
SolutionThe circle has a radius of 10 feet which is half of 20 feet. The green line is the diameter of the circle. To find the area of this circle, the formula used is $\bf{A= \pi r^2}$.
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The distance from the centre of the circle to the outside is known as the radius and in this circle is 5 feet. Therefore the diameter is double, or 10 feet. Using the formula $A= \pi r^2$, the $5$ can replace the $r$ in the formula like this, $\bf{A= \pi (5^2)}$. This can then be calculated to find the approximate area of the circle.

Determine the steps used to find the area of a circle.
HintsThe radius of a circle, $r$ is the distance from the centre of a circle to the outside.
After squaring the radius ($r=4$) in the formula,
$A=4^2 \pi$
$A=16 \pi$
multiply the value by $\pi$ using the button for $\pi$ on a calculator.
$16 \times \pi = 50.2654825...$
When a number is squared ($g^2$), it means that the number is multiplied by itself ($g \times g$).
For example, $8^2=8\times 8$.
When giving a value of a 2D shape's area, square units are always used.
Some examples include: $\text{cm}^2$, $\text{m}^2$ or $\text{ft}^2$.
SolutionStep 1
Identify the formula needed to find the area of a circle.
$A=\pi r^2$
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Step 2
Determine the measurement of the circle's radius and substitute that value into the formula.
$r=5$
$A=\pi (5^2)$
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Step 3
Evaluate the radius squared.
$5^2=25$
$A=25 \pi$
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Step 4
Calculate the radius squared multiplied by $\pi$.
$25 \times \pi = 78.539...$
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Step 5
Round your answer as directed, and include the appropriate units for the area of the circle.
$A \approx 78.5 \: \text{ft}^2$

Use a formula to find the area of a circle.
HintsIf you know the radius, this value can be substituted in the formula for $r$.
To round to the nearest square inch, it is the same as rounding to the nearest whole number.
Remember to be sure to include your squared units at the end of the measurement.
Instead of $\text{in}$, use $\text{in}^2$.
Here is a an example of how to find the area if we had a radius of 11 cm.
$r = 11\:\text{cm}$
$A = \pi r^2$
$A= \pi (11^2)$
$A \approx 380.132711...$
$A \approx 380 \:\text{cm}^2$
Solution$r = 12\:\text{in}$
$A = \pi r^2$
$A= \pi (12^2)$
$A \approx 452.389342...$
$A \approx 452 \:\text{in}^2$

Apply the formula for the area of a circle.
HintsThe $A$ is the area, and the $r$ is the radius of the circle.
This example shows the steps taken to find the area of a circle.
Radius: The radius of a circle is like a straight line going from the very centre of the circle to its edge. It's like the distance from the middle of a pizza to its crust.
Diameter: The diameter of a circle is a straight line that goes from one side of the circle to the other side, passing through the centre. It's like measuring a pizza straight across from one edge to the opposite edge. The diameter is always two times longer than the radius.
Always be sure to check if you are given the radius or the diameter.
If given the diameter, you need to halve the value to find the radius.
For example, if we were given a diameter of 22 inches, half of 22 is 11, so the radius is 11 inches.
Solution$r=8\:\text{cm}$ ; $A \approx 201.1\:\text{cm}^2$
$r=4\:\text{cm}$ ; $A \approx 50.3\:\text{cm}^2$
$d=8.5\:\text{cm}$ ; $A \approx 56.7\:\text{cm}^2$
$d=9\:\text{cm}$ ; $A \approx 63.6\:\text{cm}^2$

Understand how to apply the formula for the area of a circle.
HintsThis is the formula used to find the area of a circle.
$A$ = Area
$r$ = radius
$r^2$ means $r \times r$
For example if $r=3$, $3^2 = 9$.
The radius in this circle is $15\:m$.
The formula used to find the area would be $A=\pi \times 15^2$.
Solution$r=9$ metres
$A= \pi (r^2)$
Substitute the value for $r$ into the formula.
$A= \pi (9^2)$

Demonstrate your knowledge for calculating the area of a circle.
HintsThe formula for the area of a circle is $A=\pi r^2$.
$r$ = radius
$A$ = Area
For this example, we only need the area of the doughnut shape. Subtraction would be helpful to remove the area not needed from the hole of the doughnut.
Steps to find the area of the doughnut:
 Find the area of the large circle.
 Find the area of the smaller inner circle.
 Subtract them to find just the area of what is asked.
SolutionArea of the entire circle:
$A=\pi (7^2)$
$A \approx 154\:\text{cm}^2$
Area of the inner circle:
$A=\pi (2^2)$
$A \approx 13\:\text{cm}^2$
Subtract the values to just find what is being asked.
$154  13 = 141$
$A \approx 141 \:\text{cm}^2$