6th Grade Math Textbook, Progress

Circumference

13-12

The distance around a circle is called

the circumference (C) of the circle.

Lynn uses a string to measure around the

circle at the right and uses a metric ruler

to measure the length of the string. Then

she finds the ratio of circumference to

diameter.

circ

d

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ia

m

m

fe

e

r

t

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e

n

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ce


 



1

5

5

0

7

m

m

m

m



3.14

For every circle, the ratio of the circumference, C,

to the length of the diameter (d) is close to

3.14. Mathematicians use the Greek letter (pi)

to name this ratio. is a nonterminating,

nonrepeating decimal and is an irrational number.

You can use formulas to find the approximate

circumference of a circle.

To find circumference when the To find circumference when the

length of a diameter, d, is given: length of a radius, r, is given:

Use C d. Use C 2 r.

First estimate: First estimate:

C
 3 6 m
18 m C
 3 (2 3 yd)
18 yd

Then solve. Then solve.

C   d C 2   r

C
3.145.5 m C
 2 3.143 yd

C
17.27 m C
18.84 yd

diameter (d) = 50 mm

5.5 m 3 yd

Think


3.141592653589793...

Think

d
 2 ror

rd 2

Use   3

to estimate.

Think

17.27 m is close

to the estimate

of 18 m.

Think

18.84 yd is close

to the estimate

of 18 yd.

1.1

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