The circumference of a circle - Teachit Maths · PDF fileThe circumference of a circle...

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The circumference of a circle

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1 1½ 2 2½

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The circumference of a circle

Diameter

Approximately how many diameters fit all the way round the

circumference?

Choose your answer:

3½ 3

2

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The circumference of a circle

Diameter

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2

3

Approximately 31/7 diameters fit around the circumference.

The actual figure is:3.14159265358...

This number is called π (pi).

Circumference = π × diameterC = πd

Or using the radius;C = 2πr

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1/7

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The circumference of a circle

This rule is the same for all circles!

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The circumference of a circle

Examples: Find the circumference of these circles.

C = πd

C = 2πr

8cm 9.5cm

C = πd

C = π × 8

C = 25.1cm (1 d.p.)

C = πd

C = π × 9.5

C = 29.8cm (1 d.p.)

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The circumference of a circle

Examples: Find the circumference of these circles.

C = πd

C = 2πr

C = 2πr

C = 2 × π × 2.1

C = 13.2m (1 d.p.)

C = 2πr

C = 2 × π × 3

C = 18.8mm (1 d.p.)

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The circumference of a circle

Examples: Find the circumference of the tyre and the steering wheel.

C = πd

C = 2πr

C = πd

C = π × 23

C = 72.3cm (1 d.p.)

C = 2πr

C = 2 × π × 7.5

C = 47.1cm (1 d.p.)

23cm

7.5cm

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The circumference of a circle

Examples: Find the perimeters of these shapes.

C = πd

C = 2πr

8cm

9.5cm

Perimeter = ½ × Circumference + diameter

P = ½πd + d

P = ½π × 8 + 8

C = 20.6cm (1 d.p.)

P = ½πd + d

P = ½π × 9.5 + 9.5

C = 24.4cm (1 d.p.)

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The circumference of a circle

Examples: Find the perimeters of these shapes.

C = πd

C = 2πr

6cm8.5cm

P = ¼ × 2πr + 2r

P = ¼ × 2π × 6 + 2 × 6

C = 21.4cm (1 d.p.)

P = ¾ × 2πr + 2r

P = ¾ × 2π × 8.5 + 2 × 8.5

C = 57.1cm (1 d.p.)

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The circumference of a circle

Examples:

C = πd

C = 2πr

C = πd

d = C/π

d = 25/π

d = 8.0cm (1 d.p.)

C = 2πr

r = C/2π

r = 30/2π

r = 4.8cm (1 d.p.)

C = 30cm. Find the radius.

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C = 25cm. Find the diameter.