The Circle

1. Know the names of a circle’s features2. Calculate the circumference

Why am I doing this?

A wheel is a circle!

Circles in design – Mickey Mouse is made from circles

A real favourite SAT and GCSE question

OK - What have I got to do?

Name these Features

The distance from the centre to the edge

The distance from one side to the other passing through the centre

The distance all of the way round the edge

The blue line

The distance from the centre to the edge RADIUS

The distance from one side to the other passing through the centre DIAMETER

The distance all of the way round the edge CIRCUMFERENCE

The blue line CHORD

Sector

Segment

APPROXIMATELY FINDING THE

CIRCUMFERENCE

Level 5

APPROXIMATELY what is the relationship

(connection) between a circle’s diameter and its

circumference?

To APPROXIMATELY find the CIRCUMFERENCE MULTIPLY the

DIAMETER by 3 (C = 3 x d)Radius Diameter Circumference

4

8

12

10

5

15

18

30

42

To APPROXIMATELY find the CIRCUMFERENCE MULTIPLY the

DIAMETER by 3 (C = 3 x d)Radius Diameter Circumference

2 4 12

4 8 24

6 12 36

10 20 60

5 10 30

15 30 90

3 6 18

5 10 30

7 14 42

OAA

• A circle has a diameter of 10 cm. APPROXIMATELY (ROUGHLY), what is its circumference?

• A circle has a circumference of 18 cm. Approximately, what is its diameter?

30 cm

6 cm

Calculate the Circumference Using the Correct Formula

Level 6

Diameter = 12 cm

C = d

C = 3 X 12

C = 36

How to calculate the circumference

The symbol is the Greek letter pi. It stands for a number

that can never be found exactly. It is approximately

3.14

Evaluate the CIRCUMFERENCE

Always, write the formula (rule)

Diameter = ?cm

C = d

d = C ÷

d = C ÷ 3

d = 40 ÷ 3

d = 13.333333

How to calculate the diameter from the circumference

If the circumference is 40 cm. evaluate the DIAMETER

Always, write the formula (rule)

Diameter Radius Circumference

1 24

2 14

3 17

4 30

5 22

6 120

7 78

8 88

9 120

10 340

Rememberd = 2 X rr = d ÷ 2

Diameter Radius Circumference

1 24 12 75.36

2 14 7 43.96

3 34 17 106.76

4 60 30 188.4

5 22 11 69.08

6 120 60 376.8

7 156 78 489.84

8 176 88 552.64

9 38.22 19.11 120

10 108.28 54.14 340