9-8 circles and circumference warm up warm up lesson presentation lesson presentation problem of the...
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9-8 Circles and Circumference
Warm UpWarm Up
Lesson PresentationLesson Presentation
Problem of the DayProblem of the Day
Lesson QuizzesLesson Quizzes
9-8 Circles and Circumference
Warm UpThe length and width of a rectangle are each multiplied by 5. Find how the perimeter and area of the rectangle change.
The perimeter is multiplied by 5, and the area is multiplied by 25.
9-8 Circles and Circumference
Problem of the Day
When using a calculator to find the width of a rectangle whose length one knew, a student accidentally multiplied by 20 when she should have divided by 20. The answer displayed was 520. What is the correct width?1.3
9-8 Circles and Circumference
Learn to identify the parts of a circle and to find the circumference of a circle.
9-8 Circles and Circumference
Vocabulary
circlecenterchorddiameterradius (radii)circumferencepi
9-8 Circles and Circumference
A circle is the set of all points in a plane that are the same distance from a given point, called the center.
Center
9-8 Circles and Circumference
Radius (plural: radii). A line segment with one endpoint at the center of the circle and the other endpoint on the circle.
CenterRadius
9-8 Circles and Circumference
A diameter is a chord that passes through the center of the circle. Diameters are the longest chords in a circle.
CenterRadius
Diameter
9-8 Circles and Circumference
A chord is a line segment that has both endpoints on the circle.
CenterRadius
Diameter
Chord
9-8 Circles and Circumference
Additional Example 1: Naming Parts of a Circle
Name the circle, a diameter, and three radii.
NThe circle is circle Z.
LM is a diameter.
LN and LM are chords.
M
ZL
ZL, ZM, and ZN are radii.
9-8 Circles and Circumference
Check It Out: Example 1
Name the circle, a diameter, and three radii.
The circle is circle D.
DI, DG, and DH are radii.
G
H
DI
IG is a diameter.
IG and IH are chords.
9-8 Circles and Circumference
The distance around a circle is called the circumference.
CenterRadius
Diameter
Circumference
9-8 Circles and Circumference
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.”
Cd
Cd
=
9-8 Circles and Circumference
The formula for the circumference of a circle is C = d, or C = 2r.
The decimal representation of pi starts with 3.14159265 . . . and goes on forever without repeating. Most people estimate using either 3.14 or .22
7
9-8 Circles and Circumference
Additional Example 2: Application
A skydiver is laying out a circular target for his next jump. Estimate the circumference of the target by rounding to 3.
C = d
C 3 • 8
C 24 ft
Write the formula.
Replace with 3 and d with 8.
8 ft
9-8 Circles and Circumference
Check It Out: Example 2
A second skydiver is laying out a circular target for his next jump. Estimate the circumference of the target by rounding to 3.
C = d
C 3 • 14
C 42 yd
Write the formula.
Replace with 3 and d with 14.
14 yd
9-8 Circles and Circumference
Additional Example 3A: Using the Formula for the Circumference of a Circle
Find the missing value to the nearest hundredth. Use 3.14 for pi.
d = 11 ft; C = ?
C = d
C 3.14 • 11
C 34.54 ft
Write the formula.
Replace with 3.14 and d with 11.
11 ft
9-8 Circles and Circumference
Additional Example 3B: Using the Formula for the Circumference of a Circle
Find each missing value to the nearest hundredth. Use 3.14 for pi.
r = 5 cm; C = ?
C = 2r
C 2 • 3.14 • 5
C 31.4 cm
Write the formula.
Replace with 3.14 and r with 5.
5 cm
9-8 Circles and Circumference
Additional Example 3C: Using the Formula for the Circumference of a Circle
Find each missing value to the nearest hundredth. Use 3.14 for pi.
C = 21.98 cm; d = ?
C = d
21.98 3.14d
7.00 cm d
Write the formula.
Replace C with 21.98 and with 3.14.
21.98 3.14d_______ _______
3.14 3.14 Divide both sides by 3.14.
9-8 Circles and Circumference
Check It Out: Example 3A
Find the missing value to the nearest hundredth. Use 3.14 for pi.
d = 9 ft; C = ?
C = d
C 3.14 • 9
C 28.26 ft
Write the formula.
Replace with 3.14 and d with 9.
9 ft
9-8 Circles and Circumference
Check It Out: Example 3B
Find each missing value to the nearest hundredth. Use 3.14 for pi.
r = 6 cm; C = ?
C = 2r
C 2 • 3.14 • 6
C 37.68 cm
Write the formula.
Replace with 3.14 and r with 6.
6 cm
9-8 Circles and Circumference
Check It Out: Example 3C
Find each missing value to the nearest hundredth. Use 3.14 for pi.
C = 18.84 cm; d = ?
C = d
18.84 3.14d
6.00 cm d
Write the formula.
Replace C with 18.84 and with 3.14.
18.84 3.14d_______ _______
3.14 3.14 Divide both sides by 3.14.
9-8 Circles and Circumference
Standard Lesson Quiz
Lesson Quizzes
Lesson Quiz for Student Response Systems
9-8 Circles and Circumference
Lesson Quiz
Find the circumference of each circle. Use 3.14 for .
1. 2.
3. Find the circumference of a circle with a
diameter of 20 feet. Use 3.14 for .
C = 25.12 in. C = 18.84 in.
8 in.
62.8 ft
3 in.
9-8 Circles and Circumference
1. Identify the circumference of the given circle. Use 3.14 for .
A. 31.14 in. B. 31.21 in.
C. 31.33 in. D. 31.4 in.
Lesson Quiz for Student Response Systems
10 in.
9-8 Circles and Circumference
2. Identify the circumference of the given circle. Use 3.14 for .
A. 37.54 in. B. 37.68 in.
C. 37.81 in. D. 37.93 in.
Lesson Quiz for Student Response Systems
6 in.
9-8 Circles and Circumference
3. Identify the circumference of a circle with a diameter of 26 feet. Use 3.14 for .
A. 81.64 ft B. 81.73 ft
C. 81.86 ft D. 81.92 ft
Lesson Quiz for Student Response Systems