1. Find the Area of a Circle
We are Learning to……
Find the Area of a Circle

2. Investigation
Using the fraction circles (thirds and above) can you arrange the pieces of each type so that the pieces have the radii touching but the points are at opposite ends.
What do you notice about the shapes? What happens when you use quarters, fifths, sixths etc…?

3. Formula for the area of a circle
We can find the area of a circle using the formula
Area of a circle = π × r × r
radius or
Area of a circle = πr2

4. The circumference of a circle
Use π = 3.14 to find the area of this circle.
4 cm
A = πr2
= 3.14 × 4 × 4
= cm2

5. Finding the area given the diameter
The radius of a circle is half of its radius, or
r = d 2
We can substitute this into the formula
A = πr2
to give us a formula to find the area of a circle given its diameter.
A =
πd2 4

6. The area of a circle
Use π = 3.14 to find the area of the following circles:
2 cm
10 m
A = πr2
A = πr2
= 3.14 × 22
= 3.14 × 52
= cm2
= 78.5 m2
23 mm
Explain that rather than use the formula on the previous slide, it is usually easier to halve the diameter mentally to give the radius, before substituting it into the formula.
The most common error is to neglect to half the diameter to find the radius and to substitute this value into the formula. Ensure that pupils do not make this mistake.
78 cm
A = πr2
A = πr2
= 3.14 × 232
= 3.14 × 392
= mm2
= cm2

7. Find the shaded area to 2 decimal places.
Area problem
Find the shaded area to 2 decimal places.
Area of the square = 12 × 12
= 144 cm2 1 4
Area of sector = × π × 122
= 36π
Discuss how this area could be calculated before revealing the solution.
Shaded area = 144 – 36π
12 cm
= 30.9 cm2 (to 1 d.p.)

8. Area of a Circle Circles 2 #s 1a-e, 3a-e, 8, 9 & 11aef
The Area of a circle can be worked out by using the formula:
A = πr²
Where r is the radius and π = 3.14 to 2 decimal places
Circles 2 #s 1a-e, 3a-e, 8, 9 & 11aef