9-4
MAIN IDEA
I will identify, describe,
and classify angles.
Angles
Brent’s teacher assigned ten
problems for homework. Brent
started his homework at
4:00 P.M. He completed it at
the time shown. How far has
the minute hand turned?
10
9
8
2 1
11 12
2
3
7 6 5
New Vocabulary
angle
right angle
acute angle
obtuse angle
Math Online
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• Self-Check Quiz
An angle is a figure made from two
rays that have the same endpoint.
Angles are measured in degrees (°).
endpoint
Turns and Angles
90°
Key Concepts
1
_
turn
1
_
turn
4
3
_
turn
4
180°
2
full turn
360°
270°
Real-World EXAMPLE
Turns and Angles
1 MEASUREMENT Refer to the clock above. Write how far
the minute hand has turned in degrees and as a fraction.
Compare the angle shown on the clock to the angles shown
in the Key Concepts box.
1
So, the angle shown on the clock is 90° or a _ turn.
4
368
Chapter 9 Identify and Describe Geometric Figures
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Types of Angles
Key Concepts
A right angle measures 90°.
A right angle is formed by
perpendicular lines.
A corner, like the
corners on a desk, is a
right angle.
This symbol
means right
angle.
An acute angle measures greater
than 0° and less than 90°.
An obtuse angle measures greater
than 90°, but less than 180°.
EXAMPLES
Classify an Angle
Classify each angle as right, acute, or obtuse.
2
3
The angle is 90°.
So, it is a right angle.
The angle is greater than 90°
and less than than 180°.
It is an obtuse angle.
Write the measure of each angle in degrees and as a fraction. See Example 1 (p. 368)
1.
2.
3.
Classify each angle as right, acute, or obtuse. See Examples 2 and 3 (p. 369)
4.
7.
5.
6.
Describe what makes each type of angle an acute, obtuse,
or right angle.
Lesson 9-4 Angles 369
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)# E
# T4 IC
EXTRA P
!C
2A
0R
See page R24.
R26.
Write the measure of the angle in degrees and as a fraction. See Example 1 (p. 368)
8.
9.
10.
Classify each angle as right, acute, or obtuse. See Examples 2 and 3 (p. 369)
11.
12.
13.
14.
15.
16.
17. The timer is set to
30 minutes. How many
degrees will the dial
have turned when the
timer goes off?
18. Classify the angle shown on the gas
gauge below.
Real-World PROBLEM SOLVING
Geography
A compass can be used to find direction.
The arrow on a compass always faces north.
19. If you are facing north and turn west, what angle could
be drawn to represent your movement?
20. You are facing east and are told to turn 180°. What
direction will you be facing? Write the angle your body
has turned as a fraction.
21. OPEN ENDED Draw three different acute angles.
22.
370
Choose three objects in your classroom that
have angles. Describe how to classify each angle as acute, obtuse, or right.
Chapter 9 Identify and Describe Geometric Figures
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