The circumference of a circle is the distance around the
circle. That is, the circumference of a circle is its perimeter.
Activity 11.1
Equipment: Compass, ruler,
thread and calculator.
Purpose:
To find a formula for the circumference of a circle.
Step 1: Use the compass to draw a circle.
Step 2: Use the thread and ruler to measure the
circumference, C, of the circle drawn.
Step 3: Use the ruler to measure the diameter, d, of
the circle.
Step 4: Calculate the ratio
C : d. That is:
Activity 11.2
Step 1: Use the compass to draw a larger circle than the one drawn in
Activity 1.
Step 2: Use the thread and ruler to measure the circumference, C,
of the circle drawn.
Step 3: Use the ruler to measure the diameter, d, of the
circle.
Step 4: Calculate the ratio
C : d. That is:
In general:
For a circle of any size:
If r is the radius of the circle, then d = 2r.
Thus, the circumference, C, of a circle is given by:
Example 13
Find the circumference of a circle of diameter 63 cm using π
= 3.14.
Solution:
So, the circumference of the circle is 197.82 cm.
Note:
To find the circumference of a circle, draw a diagram and label it.
Then
write the formula
C = πd or C =
2πr. Then substitute the
given value of d or r and use a calculator to compute
the
value of C. Finally, state your answer in words.
Example 14
Find the circumference of a circle of radius 45 m using π
= 3.14.
Solution:
So, the circumference of the circle is 282.6 m.
Key Terms
circumference
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