10–2 Circles and Circumference - My Math 6
10?2 Circles and Circumference
MAIN IDEA
? Estimate and find the circumference of circles.
KEY CONCEPT
Radius and Diameter The diameter d of a circle is twice its radius r. The radius r of a circle is half of its diameter d.
BUILD YOUR VOCABULARY (pages 241?242)
A circle is the set of all
in a plane that are the
same distance from a
called the center.
A chord is any segment with both
on the circle.
The diameter is the distance its center.
a circle through
The radius is the distance from the point on a circle.
to any
The circumference is the distance
a circle.
EXAMPLE Find the Radius
The diameter of a circle is 48 centimeters. Find the radius.
r =
Write the formula.
r = r = The radius is
Replace d with 48. Divide. centimeters.
Check Your Progress The radius of a circle is 22 centimeters. Find the diameter.
Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Math Connects, Course 1 245
10?2
KEY CONCEPT
Circumference The circumference of a circle is equal to times twice its radius.
EXAMPLES Estimate the Circumference Estimate the circumference of each circle.
C =
??
C
?
C
m
Circumference of a circle
Replace with
and
d with
.
Multiply.
C =
?x?
C ? ?
C
mm
Circumference of a circle
Replace with
and r with
.
Multiply.
Check Your Progress Estimate the circumference of
each circle.
a. diameter = 4 yd
b. radius = 12 in.
Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
EXAMPLE Use a Calculator to Find Circumference
Use a calculator to find the circumference
of the circle. Round to the nearest tenth.
???`
C =
Circumference of
a circle
C =
?
?
Replace r with 3.
2 P 16 ENTER 18.8495559215
The circumference is about
yards.
246 Math Connects, Course 1
10?2
Check Your Progress Use a calculator to find the circumference of a circle with a diameter of 24 centimeters. Round to the nearest tenth.
Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
EXAMPLE
TEST EXAMPLE Anna knows the diameter of a basketball hoop but would like to find the circumference. Which method can she use to find the circumference of the basketball hoop?
A Divide the diameter by . B Multiply the radius by . C Multiply the diameter by 2, and then multiply by . D Multiply the diameter by .
Read the Item You need to determine the method used to find the circumference of the basketball hoop. You know the
of the basketball hoop.
Solve the Item
Use the formula for the circumference of a circle C =
.
The formula states that the circumference of a circle is equal to
. So, the answer is
.
HOMEWORK ASSIGNMENT
Page(s): Exercises:
Check Your Progress MULTIPLE CHOICE A standard baseball has a circumference of 9 inches. Which method can be used to find the radius of the baseball?
F Divide the circumference by and then multiply by 2.
G Divide the circumference by and then divide by 2.
H Multiply the circumference by and then multiply by 2.
J Multiply the circumference by and then divide by 2.
Math Connects, Course 1 247
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