6-9 Geometry: Circles and Circumference

Today you will learn to calculate the circumference of circles.

New Vocabulary

Circle: set of all points in a plane that are the same distance from a given point called the center.

Diameter(d): the distance across a circle through it’s center.

Radius (r): the distance from the center to any point on a circle.

Circumference (C): the distance around a circle.

Circles

.

Center

CirclesRadius

CirclesDiameter

CirclesThe diameter of a circle is 2 times

the radius.

Circumference of a CircleThe circumference of a circle is equal

to it’s diameter (d) times π , or 2 times it’s radius (r) times π.

An approximation for π is 3.14.d=2r

CirclesExample: Ferris WheelsFind the circumference of the Ferris

wheel that measures 450 ft across.

CirclesAlways start by writing the formula

that you are going to use. C=πd C≈3.14(450) C≈1,413So, the distance around the Ferris

wheel is about 1,413 feet.

Replace π with 3.14 and d with 450

Find the Circumferencer=10 in

Find the Circumferencer=10 inC=2πr

C≈2(3.14)(10) C≈62.8 in

Find the CircumferenceYou can also use

for π.Find the circumference of a circle

with radius 21 cm.

227

CircumferenceC=2πr

C≈2( )( ) C≈ 132 cm

227

211

1

3

CirclesPractice (Round to tenths if

needed) 1.diameter = 5 in2. radius = 12 m3. radius = 1

34

HomeworkPractice workbook Lesson 6-9

Be sure to show all of your work step by step. This means to write down the formula and show the values you substitute before you multiply. Don’t forget the units in

your answer.No work = No credit