The Circle part 1

1
The Circlepart 1
2
The RADIUS

• The radius is a line drawn from the centre of the
  circle to the outside edge

3
The DIAMETER

• The diameter is a line drawn from one side of the
  circle to the other side through the centre

4
Radius Diameter

• The diameter is 2 times the radius

• d 2r

radius
radius
radius
radius

• The radius is half the diameter

diameter

• r 1 d
• 2

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The CIRCUMFERENCE

• The circumference of a circle is the distance
  around the outside of the circle

Circumference
Circumference
Circumference
Circumference
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All Together
d 2r

• r 1 d
• 2

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Calculating the Diameter or Radius
Calculate the diameters of the following
circles a) r 3 cm b) r 35 cm c) r 102
m d) r 15.5 m e) r6.2 cm f) r 18.9 cm
r 20cm
d 40cm
Answers a) 6 cm b) 70 cm c) 204 m d)
31 m e) 12.4 cm f) 37.8 cm
d 6cm
Calculate the radii of the following circles a)
d14 cm b) d 25 cm c) d 32 m d) d74 m
e) d4.8 cm f) d 25.3 m
r 3cm
Answers a) 7 cm b) 12.5 cm c) 16 m
d) 37 m e) 2.4 cm f) 12.65 cm
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TASK

• Draw a circle

• Use your ruler
• and measure the
• diameter

• Use your string
• and ruler to
• measure the
• circumference

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NOW DIVIDE THE LENGTH OF THE CIRCUMFERENCE BY THE
LENGTH OF THE DIAMETER
about 3
10
Pi

• When we divide the
• length of the circumference
• of any circle by its diameter
• the number we get is always
• the same.

p 3.14
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Calculating the Circumference
d
We can rearrange this formula to produce a new
formula that will allow us to calculate the
length of the circumference
All we do is move the diameter from one side of
the formula to the other.
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Calculating the Circumference

• d4 cm
• r2 cm
• Cpd
• C

3.14
x

4
.C12.56 cm
Answers a) 43.96 cm b) 87.92 m c)
141.3 m d) 21.352 cm
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Calculating the Circumference

• r5 m
• d10 m
• Cpd
• C

x
10
3.14

.C31.4 m
Answers a) 113.04 cm b) 213.52 m c)
40.192 mm d) 11.932 m
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Calculating the Circumference
Calculate the circumference of each of the
following circles
2 a) d3mm b) r8m c) r15cm
d) d126 m e) r25.5 cm f) d2.4 m
g) r26.4 m h) d6.9 cm i) d1.5 mm j)
r4.8 cm k) d0.26 m l) r0.6 m
Answers 1a) 163.28 cm b) 200.96 m c)
38.308 mm d) 17.584 cm
2a)9.42 mm b)50.24 m c)94.2 cm
d)395.64 m e)160.14 cm f)7.536 m
g)165.762 m h)21.666 cm i)4.71 mm
j)30.144cm k)0.8164 m l)3.768
m
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THE END