1. Preview
Warm Up
California Standards
Lesson Presentation

2. Warm Up
The length and width of a rectangle are each multiplied by 5. Find how the perimeter of the rectangle changes.
The perimeter is multiplied by 5.

3. California
Standards
MG1.1 Understand the concept of a constant such as p; know the formulas for the circumference and area of a circle.
Also covered: AF1.1, AF3.1, AF3.2, MG1.2

4. Vocabulary
circle
center
radius (radii)
diameter
circumference pi

5. A circle is the set of all points in a plane that are the same distance from a given point, called the center.
Center

6. Radius (plural: radii) A line segment with one endpoint at the center of the circle and the other endpoint on the circle.
Radius
Center

7. Diameter A line segment that passes through the center of the circle and has both endpoints on the circle. Notice that the length of the diameter is twice the length of the radius, d = 2r.
Radius
Center
Diameter

8. Circumference The distance around a circle.
Radius
Center
Diameter

9. Additional Example 1: Naming Parts of a Circle
Name the circle, a diameter, and three radii. L Z M N
The center is point Z, so this is circle Z.
LM is a diameter.
ZL, ZM, and ZN are radii.

10. Name the circle, a diameter, and three radii.
Check It Out! Example 1
Name the circle, a diameter, and three radii. G H D I
The center is point D, so this is circle D.
IG is a diameter.
DI, DG, and DH are radii.

11. The ratio of the circumference to the diameter, , is the same for any circle. This
ratio is represented by the Greek letter , which is read “pi.” C d
The decimal representation of pi starts with and goes on forever without repeating. Most people approximate p using
either or 22 7

12. Because , you can
multiply both sides of the equation by d to get a formula for circumference. You can also substitute 2r for d because d = 2r. C d
=  C d
=  C d
· d =  · d
C = d
C = (2r) = 2pr

13. Additional Example 2: Application
A skydiver is laying out a circular target for his next jump. Estimate the circumference of
the target by using as an estimate for p.
22 7
10.5 ft
C = d
Use the formula.
22 7
C  • 10.5
22 7
Replace  with and d with 10.5.
C  33 ft
The circumference of the target is about 33 feet.

14. chalk design by using as an estimate for p.
Check It Out! Example 2
A concrete chalk artist is drawing a circular design. Estimate the circumference of the
chalk design by using as an estimate for p.
22 7
14 in.
C = d
Use the formula. 2
22 7
C  • 14
22 7
Replace  with and d with 14. 1
C ≈ 44 in.
The circumference of the chalk design is about
44 inches.

15. Use as an estimate for  when the
diameter or radius is a multiple of 7. 22 7
Helpful Hint

16. Additional Example 3: Using the Formula for the Circumference of a Circle
A. Find the missing value to the nearest hundredth. Use 3.14 as an estimate for p.
d = 11 ft; C = ?
11 ft
C = d
Write the formula.
C  3.14 • 11
Replace  with 3.14 and d with 11.
C  ft

17. Additional Example 3: Using the Formula for the Circumference of a Circle
B. Find each missing value to the nearest hundredth. Use 3.14 as an estimate for p.
r = 5 cm; C = ?
5 cm
C = 2r
Write the formula.
C  2 • 3.14 • 5
Replace  with 3.14 and r with 5.
C  cm

18. Check It Out! Example 3
A. Find the missing value to the nearest hundredth. Use 3.14 as an estimate for p.
d = 9 ft; C = ?
9 ft
C = d
Write the formula.
C  3.14 • 9
Replace  with 3.14 and d with 9.
C  ft

19. Check It Out! Example 3
B. Find each missing value to the nearest hundredth. Use 3.14 as an estimate for p.
r = 6 cm; C = ?
6 cm
C = 2r
Write the formula.
C  2 • 3.14 • 6
Replace  with 3.14 and r with 6.
C  cm

20. Replace C with 18.84 and  with 3.14.
Check It Out! Example 3
C. Find each missing value to the nearest hundredth. Use 3.14 as an estimate for p.
C = cm; d = ?
C = d
Write the formula.
 3.14d
Replace C with and  with 3.14. d
_______ 
Divide both sides by 3.14.
6.00 cm  d

21. Lesson Quiz
Find the circumference of each circle. Use 3.14 as an estimate for .
3. Find the circumference of a circle with diameter of 20 feet. Use 3.14 as an estimate t for .
C ≈ in.
C ≈ in.
C ≈ 62.8 ft