The circumference of a circle
Approximately how many diameters
fit all the way round the
circumference?
The circumference of a circle
Choose your answer:
1
Diameter
3½
3
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1
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The circumference of a circle
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2
1½
2½
4
2
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The circumference of a circle
Approximately 31/7 diameters
fit around the circumference.
1
The actual figure is:
3.14159265358...
2
Diameter
1/
7
This number is called π (pi).
This rule is the same
for all circles!
Circumference = π × diameter
C = πd
Or using the radius;
C = 2πr
3
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3
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The circumference of a circle
Examples:
Find the circumference of these circles.
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4
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The circumference of a circle
C = πd
Examples:
C = 2πr
Find the circumference of these circles.
C = 2πr
9.5cm
8cm
C = πd
C = πd
C = 2πr
C = 2πr
C=π×8
C = π × 9.5
C = 2 × π × 2.1
C=2×π×3
C = 25.1cm (1 d.p.)
C = 29.8cm (1 d.p.)
C = 13.2m (1 d.p.)
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C = πd
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5
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C = 18.8mm (1 d.p.)
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1
The circumference of a circle
Examples:
Find the circumference of the tyre and
the steering wheel.
The circumference of a circle
C = πd
Examples:
C = 2πr
Find the perimeters of these shapes.
C = πd
C = 2πr
8cm
23cm
9.5cm
Perimeter = ½ × Circumference + diameter
7.5cm
C = πd
C = 2πr
P = ½πd + d
P = ½πd + d
C = π × 23
C = 2 × π × 7.5
P = ½π × 8 + 8
P = ½π × 9.5 + 9.5
C = 47.1cm (1 d.p.)
C = 72.3cm (1 d.p.)
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The circumference of a circle
Examples:
Find the perimeters of these shapes.
C = 24.4cm (1 d.p.)
C = 20.6cm (1 d.p.)
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The circumference of a circle
C = πd
Examples:
C = 2πr
C = πd
C = 2πr
8.5cm
6cm
C = 25cm. Find the diameter.
P = ¼ × 2πr + 2r
P = ¾ × 2πr + 2r
P = ¼ × 2π × 6 + 2 × 6
P = ¾ × 2π × 8.5 + 2 × 8.5
C = 21.4cm (1 d.p.)
C = 57.1cm (1 d.p.)
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C = 30cm. Find the radius.
C = πd
C = 2πr
d=
C/
r = C/2π
d=
25/
π
r=
π
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2π
r = 4.8cm (1 d.p.)
d = 8.0cm (1 d.p.)
9
30/
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2