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140

ANSWERS

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170

ANSWERS

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60

ANSWERS

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40

ANSWERS

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30

ANSWERS

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55

ANSWERS

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45

ANSWERS

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80

ANSWERS

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540

ANSWERS

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Circumference and circle

Objective: after this lesson you will be able

to calculate the length of a circumference

to calculate the area of a circle

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Can you remember the names for these elements?

Circumference(the red line)

Centre

Radius

Diameter(its length is two times

the radius)

Chord

Arc

(the blue line)

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Circumferences is the perimeter of this shape

Try to measure some circumferences.

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Blue

PurpleYellow

Green

Red

Diameter

d

Circumference

C

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This is a special number:

= 3.141592653589793238462643383279502

You need to remember just

= 3.14

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To know the length of this circumference

you need to know

its diameter d.

The length of the

circumference isC=

So, the length of this circumference is:

C= = 3.14 12 = 37.68 ( 2 . . )

12 cm

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Calculate the length of this circumference

20 cm

Answer: 62.8 cmHint: C=

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Calculate the length of this circumference

15 cm

Answer: 47.1 cmHint: C=

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Calculate the length of this circumference

3 m

Answer: 18.84 mHint: C=

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Calculate the length of this circumference

5 mm

Answer: 31.4 mmHint: C=

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The area inside a circumference is called circle

Try to measure some circles.

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Blue

Purple

Yellow

Green

Red

Radius

r

Circle

C

r

Radius squared

r

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Again the special number:

= 3.14

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To know the area of this circle

you need to know

its radius r.

The area of the

circle is

=

So, the area of this circle is:

= = 3.14 12 = 452.16 (to 2 d.p.)

12 cm

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Calculate the area of this circle

15 cm

Answer: 706.5

Hint:A=

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Calculate the area of this circle

6.5 cm

Answer: 132.66

Hint:A=

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Calculate the area of this circle

18 cm

Answer: 254.34

Hint:A=

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Calculate the area of this circle

2.5 cm

Answer: 4.91

Hint:A=

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Calculate the length of the blue arc of

circumference

20 cm

Answer: 31.4 cm

C=

2 0 == 62.8 cm

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Calculate the length of the blue arc of

circumference

11 cm

Answer: 17.27 cmHint: C=

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Calculate the area of this semicircle

Answer: 2.45

2.5 cm

Hint:A=

W k h

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Worksheet