1. Circle parts
This slide simply gives the definitions of the terminology – the following slides expand on these definitions, with corresponding diagrams. The terms may be drag-and-dropped into the correct position. If a term is incorrectly placed, it will snap back to the original position, otherwise it will snap into place. F

2. Area of a circle
This animation shows how the area of a circle can be approximated to the area of a parallelogram of base length r and height r.
Watch the circle pieces rearrange into an approximate parallelogram an ask a volunteer to use the pen tool to label the length and the height in terms of r.
Deduce from this that the area of a circle is r2.

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

5. Find the area of this shape
Use π = 3.14 to find area of this shape.
The area of this shape is made up of the area of a circle of diameter 13 cm and the area of a rectangle of width 6 cm and length 13 cm.
Area of circle = 3.14 × 6.52
13 cm
6 cm
= cm2
Area of rectangle = 6 × 13
Compare this with slide 74, which finds the perimeter of the same shape.
= 78 cm2
Total area =
= cm2

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
23 mm
78 cm
A = πr2
A = πr2
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.

7. The circumference of a circle
For any circle,
π =
circumference
diameter
or,
π = C d
We can rearrange this to make an formula to find the circumference of a circle given its diameter.
Pupils should be asked to learn these formulae.
C = πd

8. The circumference of a circle
Use π = 3.14 to find the circumference of this circle.
C = πd
8 cm
= 3.14 × 8
= cm
Tell pupils that when solving a problem like this they should always start by writing down the formula that they are using.
This will minimize the risk of using the radius instead of the diameter, for example.

9. The circumference of a circle
Use π = 3.14 to find the circumference of the following circles:
4 cm
C = πd
C = πd
9 m
23 mm
58 cm
C = πd
C = πd
For each one, start by asking pupils what formula we have to use.
Estimate each answer first using  = 3, or use this to check the answer.

10. Find the perimeter of this shape
Use π = 3.14 to find perimeter of this shape.
The perimeter of this shape is made up of the circumference of a circle of diameter 13 cm and two lines of length 6 cm.
13 cm
6 cm
Perimeter = 3.14 ×
= cm