1. Perimeter and Circumference
Course 2
9-2
Perimeter and Circumference
Problem of the Day- Warm Up
437 tiles

2. Perimeter and Circumference
Course 2
9-2
Perimeter and Circumference C d
The distance around a circle is called circumference. For every circle, the ratio of circumference C to diameter d is the same. This ratio, , is
represented by the Greek letter , called pi. Pi is approximately equal to 3.14 or By solving the 22 7
equation =  for C, you get the formula for circumference. the formula for circumference.
C = d, or C = 2r.
Copy first sentence and the 3 equations in the last 3 lines.

3. CIRCUMFERENCE OF A CIRCLE
Course 2
9-2
Perimeter and Circumference
CIRCUMFERENCE OF A CIRCLE
The circumference C of a circle is  times the diameter d, or 2 times the radius r.
Radius
C = d, or C = 2r
Diameter
Copy the last column of the picture of a circle with the 3 parts labeled (radius, diameter, circumference)
Circumference

4. Additional Example 3A: Finding the Circumference of a Circle
Course 2
9-2
Perimeter and Circumference
Additional Example 3A: Finding the Circumference of a Circle
Find the circumference of the circle to the nearest tenth. Use 3.14 for .
C = d
You know the diameter.
12 in.
C  3.14 · 12
Substitute for  and d.
C  37.68
Multiply.
Copy into notes
The circumference of the circle is about 37.7 in.

5. Additional Example 3B: Finding the Circumference of a Circle
Course 2
9-2
Perimeter and Circumference
Additional Example 3B: Finding the Circumference of a Circle
Find the circumference of the circle to the nearest tenth. Use 3.14 for .
C = 2r
You know the radius.
18 cm
C  2 · 3.14 · 18
Substitute for  and r.
C 
Multiply.
Copy into notes
The circumference of the circle is about cm.

6. Perimeter and Circumference
Course 2
9-2
Perimeter and Circumference
Check It Out: Example 3A
Find the circumference of the circle to the nearest tenth. Use 3.14 for .
C = d
You know the diameter.
7 in.
C  3.14 · 7
Substitute for  and d.
C  21.98
Multiply.
Copy into notes
The circumference of the circle is about 22.0 in.

7. Perimeter and Circumference
Course 2
9-2
Perimeter and Circumference
Check It Out: Example 3B
Find the circumference of the circle to the nearest tenth. Use 3.14 for .
C = 2r
You know the radius.
11 cm
C  2 · 3.14 · 11
Substitute for  and r.
C  69.08
Multiply.
The circumference of the circle is about 69.1 cm.

8. Perimeter and Circumference
Course 2
9-2
Perimeter and Circumference
If the diameter or radius of a circle is a
multiple of 7, use for p.
Helpful Hint
22 7
Copy into notes

9. Additional Example 4: Application
Course 2
9-2
Perimeter and Circumference
Additional Example 4: Application
The diameter of a circular pond is 42 m. What is its circumference? Use
for . 22 7
C = d
You know the diameter. 22 7 22 7
C 
· 42
Substitute
for  and 42 for d. 22 7 42 1
C 
Write 42 as a fraction. 6
Copy into notes 22 7 42 1
C 
Simplify. 1
C  132
Multiply.
The circumference of the pond is about 132 m.

10. Perimeter and Circumference
Course 2
9-2
Perimeter and Circumference
Check It Out: Example 4
The diameter of a circular spa is 14 m. What is its circumference? Use 22 7
for .
C = d
You know the diameter. 22 7 22 7
C 
· 14
Substitute
for  and 14 for d. 22 7 14 1
C 
Write 14 as a fraction. 2 22 7 14 1
C 
Simplify. 1
C  44
Multiply.
The circumference of the spa is about 44 m.