Lesson 6-9 Pages 275-277

Geometry: Circles and

Circumference (and Area)

Lesson Check 6-8

What you will learn!

How to find the circumference of circles.

CircleCircleCenterCenterDiameterDiameterRadiusRadiusCircumferenceCircumference

What you really need to know!

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

What you really need to know!

The diameter d is the distance across the circle through its center. The radius r is the distance from the center to any point on the circle.

What you really need to know!

The circumference C is the distance around the circle.

RadiusDiamet

erChord

What you really need to know!

For every circle, C ÷ d is always equal to 3.1415926…. This number is represented by the Greek letter (pi).

What you really need to know!

We will use 3.14 for .

What you really need to know!

The circumference C of a circle is equal to its diameter d times , or 2 times its radius r times .

C = d or C = 2r

What you really need to know!

The Area A of a circle is equal to times r2.

A = r2 or A = rr

Demonstration

1.Identify radius and diameter.

2.Use d to find the Circumference.

3.Use r r to find Area.

Example 1:

Find the circumference around the hamster’s running wheel. Round to the nearest tenth.

r = 3 inches

C = 2r

C = 2 • 3.14 • 3

C = 18.8 in

Example 1:

Find the area inside the hamster’s running wheel. Round to the nearest tenth. A = r2

A = •r•r

A = 3.14 • 3 • 3

A = 28.3 in2

Example 2:

Find the circumference of a circle with a diameter of 49 centimeters.

d = 49 cm

r = 24.5 cm

C = 2r

C = 2 3.14 24.5

C = 153.86 cm

49 cm

Example 2:

Find the area of a circle with a diameter of 49 centimeters.

d = 49 cm

r = 24.5 cm

A = rr

A = 3.14 • 24.5 • 24.5

A = 1884.785 cm2

49 cm

Page 276

Guided Practice

#’s 4-10

Pages 275-276 with someone at home and study

examples!

Read:

Homework: Page 277

#’s 12-22 even, 23,

#’s 27-31

Lesson Check Ch 6

Page

579

Lesson 6-9

Study Guide and Review

Pages

278-280

#’s 1-69(Odd answers in back of book)

Prepare for Test!

Page

281

#’s 1-25 Lesson Check Ch 6

Circle intoRectangle

Prepare for Test!

Pages

282-283

#’s 1-15

A = L x W

L = ½ x 2 x x r

W = r

A = ½ x 2 x x r x r

A = x r x r

A = r2

C = •d or 2••rCircumference

AreaA = •r•r

Circumference = 2r

Area = r2 or rr

= 3.14

Radius

1. Find the LCD.2. Rewrite fractions using the LCD.3. Add only the numerators, keeping the LCD.4. Reduce to lowest terms.

The distance around a geometric figure.