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Copyright © 2005 Pearson Education, Inc.
Introduction to
Trigonometry
Angle Relationships and
Similar Triangles
Copyright © 2005 Pearson Education, Inc.
Basic Terms continued
Angle-formed by rotating
a ray around its endpoint.
The ray in its initial
position is called the
initial side of the angle.
The ray in its location
after the rotation is the
terminal side of the
angle.
Copyright © 2005 Pearson Education, Inc.
Slide 1-3
Basic Terms continued
Positive angle: The
rotation of the terminal
side of an angle
counterclockwise.
Copyright © 2005 Pearson Education, Inc.
Negative angle: The
rotation of the terminal
side is clockwise.
Slide 1-4
Standard Position
An angle is in standard position if its vertex is
at the origin and its initial side is along the
positive x-axis.
Angles in standard position having their terminal
sides along the x-axis or y-axis, such as angles
with measures 90, 180, 270, and so on, are
called quadrantal angles.
Copyright © 2005 Pearson Education, Inc.
Slide 1-5
Coterminal Angles
A complete rotation of a ray results in an angle
measuring 360. By continuing the rotation,
angles of measure larger than 360 can be
produced. Such angles are called coterminal
angles.
Copyright © 2005 Pearson Education, Inc.
Slide 1-6
Angles and Relationships
q
m
n
Name
Angles
Rule
Alternate interior angles
4 and 5
3 and 6
Angles measures are equal.
Alternate exterior angles
1 and 8
2 and 7
Angle measures are equal.
Interior angles on the same
side of the transversal
4 and 6
3 and 5
Angle measures add to 180.
Corresponding angles
2 & 6, 1 & 5,
3 & 7, 4 & 8
Angle measures are equal.
Copyright © 2005 Pearson Education, Inc.
Slide 1-7
Conditions for Similar Triangles
Corresponding angles must have the same
measure.
Corresponding sides must be proportional.
(That is, their ratios must be equal.)
Copyright © 2005 Pearson Education, Inc.
Slide 1-8
Example: Finding Angle Measures
Triangles ABC and DEF
are similar. Find the
measures of angles D
and E.
D
Since the triangles are
similar, corresponding
angles have the same
measure.
Angle D corresponds to
angle A which = 35
A
112
35
F
C
112
33
Copyright © 2005 Pearson Education, Inc.
E
Angle E corresponds to
angle B which = 33
B
Slide 1-9
Example: Finding Side Lengths
Triangles ABC and DEF
are similar. Find the
lengths of the unknown
sides in triangle DEF.
32 64
16 x
32 x 1024
x 32
D
A
16
112
35
64
F
32
C
112
33
48
Copyright © 2005 Pearson Education, Inc.
To find side DE.
B
E
To find side FE.
32 48
16 x
32 x 768
x 24
Slide 1-10
Example: Complementary Angles
Find the measure of each angle.
Since the two angles form a right
angle, they are complementary
angles. Thus,
k 20 k 16 90
k +20
k 16
2k 4 90
2 k 86
k 43
Copyright © 2005 Pearson Education, Inc.
The two angles have measures of
43 + 20 = 63 and 43 16 = 27
Slide 1-11
Example: Coterminal Angles
Find the angles of smallest possible positive
measure coterminal with each angle.
a) 1115
b) 187
Add or subtract 360 as may times as needed to
obtain an angle with measure greater than 0 but
less than 360.
o
o
o
a) 1115 3(360 ) 35
b) 187 + 360 = 173
Copyright © 2005 Pearson Education, Inc.
Slide 1-12
Example: Finding Angle Measures
Find the measure of each
marked angle, given that
lines m and n are parallel.
(6x + 4)
(10x 80)
m
n
The marked angles are
alternate exterior angles,
which are equal.
Copyright © 2005 Pearson Education, Inc.
6 x 4 10 x 80
84 4 x
21 x
One angle has measure
6x + 4 = 6(21) + 4 = 130
and the other has measure
10x 80 = 10(21) 80 =
130
Slide 1-13