NAME ___________________________________________________ DATE _____________________ PERIOD __________
Lesson 1 Reteach
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
The distance from
the center to any
point on the circle
is the radius.
r
The distance across
the circle through the
center is the diameter.
d
The circumference of a circle is the distance around the circle. In every circle, the ratio of the
circumference to the diameter is equal to approximately 3.14, represented by the Greek letter π (pi).
Circumference of a Circle
Words
The circumference C of a circle is equal to its
diameter times π, or 2 times its radius times π.
Model
r
C
d
Symbols
C = π d or C = 2π r
Copyright © The McGraw-Hill Companies, Inc. Permission is granted to reproduce for classroom use.
Example
Find the circumference of the circle. Round to the nearest tenth.
Circumference of a circle
C = 2πr
=2·π·7
Replace r with 7.
≈ 44.0
Simplify. Use a calculator.
The circumference is about 44.0 kilometers.
7 km
Exercises
Find the circumference of each circle. Round to the nearest tenth.
2.
1.
3.
9 km
4 ft
8 in.
5.
4.
8 cm
6.
2 ft
7. diameter = 5 centimeters
Math Accelerated • Chapter 12 Volume and Surface Area
5 in.
8. radius = 3 feet
NAME ___________________________________________________ DATE _____________________ PERIOD __________
Lesson 2 Reteach
Area of Circles
Area of Circles
Words
The area A of a circle in square units is equal to π
times the square of the length of its radius.
Model
r
Symbols
Example
A = πr 2
Find the area of each circle. Round to the nearest tenth.
a.
b.
30 in.
4 ft
A = πr 2
= π · (4)2
= π · 16
≈ 50.3
A = πr 2
= π · (15)2
= π · 225
≈ 706.9
Area of a circle
Replace r with 4.
2
Evaluate (4) .
Use a calculator.
Area of a circle
Replace r with 15.
Evaluate (15)2.
Use a calculator.
Exercises
Find the area of each circle. Round to the nearest tenth.
1.
2.
m
6c
3.
4.
18 in.
20 m
2 ft
Match each circle described in the column on the left with its corresponding area in the
column on the right.
5. radius = 6 units
a. 452.4 units2
6. diameter = 24 units
b. 804.2 units2
7. diameter = 50 units
c. 7854.0 units2
8. radius = 16 units
d. 113.1 units2
9. radius = 50 units
e. 2123.7 units2
10. radius = 26 units
f. 1963.5 units2
Math Accelerated • Chapter 12 Volume and Surface Area
Copyright © The McGraw-Hill Companies, Inc. Permission is granted to reproduce for classroom use.
The area is about 706.9 in2.
The area is about 50.3 ft2.