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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