# User manual | 9.3 Similar Right Triangles Essential Question MAKING ```9.3
TEXAS ESSENTIAL
KNOWLEDGE AND SKILLS
Similar Right Triangles
Essential Question
How are altitudes and geometric means of
right triangles related?
G.8.A
G.8.B
Writing a Conjecture
Work with a partner.
—
a. Use dynamic geometry software to construct right △ABC, as shown. Draw CD
so that it is an altitude from the right angle to the hypotenuse of △ABC.
5
A
4
D
3
2
1
B
0
C
0
1
2
3
4
5
6
7
8
−1
MAKING
MATHEMATICAL
ARGUMENTS
To be proficient in math,
you need to understand
and use stated assumptions,
definitions, and previously
established results in
constructing arguments.
Points
A(0, 5)
B(8, 0)
C(0, 0)
D(2.25, 3.6)
Segments
AB = 9.43
BC = 8
AC = 5
b. The geometric mean of two positive numbers a and b is the positive number x
that satisfies
a x
x is the geometric mean of a and b.
x b
Write a proportion involving the side lengths of △CBD and △ACD so that CD is
the geometric mean of two of the other side lengths. Use similar triangles to justify
your steps.
— = —.
c. Use the proportion you wrote in part (b) to find CD.
d. Generalize the proportion you wrote in part (b). Then write a conjecture about how
the geometric mean is related to the altitude from the right angle to the hypotenuse
of a right triangle.
Comparing Geometric and Arithmetic Means
Work with a partner. Use a
spreadsheet to find the arithmetic
mean and the geometric mean
of several pairs of positive
numbers. Compare the two
means. What do you notice?
1
2
3
4
5
6
7
8
9
10
11
A
a
3
4
6
0.5
0.4
2
1
9
10
B
b
C
D
Arithmetic Mean Geometric Mean
4
3.5
3.464
5
7
0.5
0.8
5
4
16
100
Communicate Your Answer
3. How are altitudes and geometric means of right triangles related?
Section 9.3
Similar Right Triangles
481
9.3 Lesson
What You Will Learn
Identify similar triangles.
Solve real-life problems involving similar triangles.
Core Vocabul
Vocabulary
larry
Use geometric means.
geometric mean, p. 484
Previous
altitude of a triangle
similar figures
Identifying Similar Triangles
When the altitude is drawn to the hypotenuse of a right triangle, the two smaller
triangles are similar to the original triangle and to each other.
Theorem
Theorem 9.6 Right Triangle Similarity Theorem
If the altitude is drawn to the hypotenuse of a
right triangle, then the two triangles formed are
similar to the original triangle and to each other.
C
A
D
△CBD ∼ △ABC, △ACD ∼ △ABC,
and △CBD ∼ △ACD.
C
Proof Ex. 45, p. 488
A
B
C
D D
B
Identifying Similar Triangles
Identify the similar triangles in the diagram.
S
U
R
SOLUTION
T
Sketch the three similar right triangles so that the corresponding angles and sides have
the same orientation.
S
T
S
T
U
R
U
R
T
△TSU ∼ △RTU ∼ △RST
Monitoring Progress
Help in English and Spanish at BigIdeasMath.com
Identify the similar triangles.
1. Q
2. E
H
T
S
482
Chapter 9
R
Right Triangles and Trigonometry
G
F
Solving Real-Life Problems
Modeling with Mathematics
A roof has a cross section that is a right triangle. The diagram shows the approximate
dimensions of this cross section. Find the height h of the roof.
Y
5.5 m
Z
3.1 m
h
W
6.3 m
X
SOLUTION
1. Understand the Problem You are given the side lengths of a right triangle. You
need to find the height of the roof, which is the altitude drawn to the hypotenuse.
2. Make a Plan Identify any similar triangles. Then use the similar triangles to
write a proportion involving the height and solve for h.
3. Solve the Problem Identify the similar triangles and sketch them.
Z
Z
Y
COMMON ERROR
Notice that if you tried
to write a proportion
using △XYW and △YZW,
then there would be two
unknowns, so you would
not be able to solve for h.
3.1 m
6.3 m
5.5 m
5.5 m
h
X
W
h
Y
W
X
3.1 m
Y
△XYW ∼ △YZW ∼ △XZY
Because △XYW ∼ △XZY, you can write a proportion.
YW XY
ZY
XZ
h
3.1
—=—
5.5 6.3
h ≈ 2.7
—=—
Corresponding side lengths of similar triangles are proportional.
Substitute.
Multiply each side by 5.5.
The height of the roof is about 2.7 meters.
4. Look Back Because the height of the roof is a leg of right △YZW and right
△XYW, it should be shorter than each of their hypotenuses. The lengths of the
two hypotenuses are YZ = 5.5 and XY = 3.1. Because 2.7 < 3.1, the answer
seems reasonable.
Monitoring Progress
Help in English and Spanish at BigIdeasMath.com
Find the value of x.
3.
4.
E
13
H 5
3
G
K
x
x
J
4
12
L
5
M
F
Section 9.3
Similar Right Triangles
483
Using a Geometric Mean
Core Concept
Geometric Mean
The geometric mean of two positive numbers a and b is the positive number x
—
a x
that satisfies — = —. So, x2 = ab and x = √ ab .
x b
Finding a Geometric Mean
Find the geometric mean of 24 and 48.
SOLUTION
x2 = ab
Definition of geometric mean
⋅
x2 = 24 48
Substitute 24 for a and 48 for b.
⋅
x = √24 ⋅ 24 ⋅ 2
—
x = √24 48
—
—
x = 24√2
Take the positive square root of each side.
Factor.
Simplify.
—
The geometric mean of 24 and 48 is 24√2 ≈ 33.9.
— is drawn to the hypotenuse, forming two smaller right
In right △ABC, altitude CD
triangles that are similar to △ABC. From the Right Triangle Similarity Theorem, you
know that △CBD ∼ △ACD ∼ △ABC. Because the triangles are similar, you can write
and simplify the following proportions involving geometric means.
C
A
D
B
C
CD
AD
BD
CD
CB
DB
—=—
⋅
CD 2 = AD BD
A
D
B
C
D
AC
AD
AB
CB
AB
AC
—=—
—=—
⋅
CB 2 = DB AB
⋅
AC 2 = AD AB
Theorems
Theorem 9.7 Geometric Mean (Altitude) Theorem
C
In a right triangle, the altitude from the right angle to
the hypotenuse divides the hypotenuse into two segments.
The length of the altitude is the geometric mean of the
lengths of the two segments of the hypotenuse.
A
D
CD2
Proof Ex. 41, p. 488
⋅
= AD BD
Theorem 9.8 Geometric Mean (Leg) Theorem
C
In a right triangle, the altitude from the right angle to the
hypotenuse divides the hypotenuse into two segments.
The length of each leg of the right triangle is the
geometric mean of the lengths of the hypotenuse
and the segment of the hypotenuse that is adjacent
to the leg.
Proof Ex. 42, p. 488
484
Chapter 9
Right Triangles and Trigonometry
B
A
⋅
⋅
D
CB 2 = DB AB
AC 2 = AD AB
B
Using a Geometric Mean
Find the value of each variable.
COMMON ERROR
In Example 4(b), the
Geometric Mean (Leg)
Theorem gives
y2 = 2 (5 + 2), not
y2 = 5 (5 + 2), because
the side with length y is
adjacent to the segment
with length 2.
⋅
⋅
a.
2
b.
x
y
6
5
3
SOLUTION
a. Apply the Geometric Mean
(Altitude) Theorem.
b. Apply the Geometric Mean
(Leg) Theorem.
⋅
x2 = 6 3
x2
= 18
y2
—
x = √ 18
—
x = √9
⋅
= 2 ⋅7
y2 = 2 (5 + 2)
y2 = 14
⋅ √2
—
—
y = √ 14
—
—
x = 3√ 2
The value of y is √ 14 .
—
The value of x is 3√ 2 .
Using Indirect Measurement
To find the cost of installing a rock wall in your schooll
gymnasium, you need to find the height of the gym wall.
all.
all
You use a cardboard square to line up the top and bottom
om
of the gym wall. Your friend measures the vertical distance
ance
from the ground to your eye and the horizontal distance
ce
from you to the gym wall. Approximate the height of
the gym wall.
w ft
8.5 ft
SOLUTION
5 ft
By the Geometric Mean (Altitude) Theorem, you know
w
that 8.5 is the geometric mean of w and 5.
⋅
8.52 = w 5
Geometric Mean (Altitude) Theorem
72.25 = 5w
Square 8.5.
14.45 = w
Divide each side by 5.
The height of the wall is 5 + w = 5 + 14.45 = 19.45 feet.
Monitoring Progress
Help in English and Spanish at BigIdeasMath.com
Find the geometric mean of the two numbers.
5. 12 and 27
x
4
6. 18 and 54
7. 16 and 18
8. Find the value of x in the triangle at the left.
9
9. WHAT IF? In Example 5, the vertical distance from the ground to your eye is
5.5 feet and the distance from you to the gym wall is 9 feet. Approximate
the height of the gym wall.
Section 9.3
Similar Right Triangles
485
Exercises
9.3
Dynamic Solutions available at BigIdeasMath.com
Vocabulary and Core Concept Check
1. COMPLETE THE SENTENCE If the altitude is drawn to the hypotenuse of a right triangle, then the two
triangles formed are similar to the original triangle and __________________.
2. WRITING In your own words, explain geometric mean.
Monitoring Progress and Modeling with Mathematics
In Exercises 3 and 4, identify the similar triangles.
(See Example 1.)
3.
F
In Exercises 11–18, find the geometric mean of the two
numbers. (See Example 3.)
E
H
G
4.
M
11. 8 and 32
12. 9 and 16
13. 14 and 20
14. 25 and 35
15. 16 and 25
16. 8 and 28
17. 17 and 36
18. 24 and 45
In Exercises 19–26, find the value of the variable.
(See Example 4.)
L
N K
19.
20.
x
In Exercises 5–10, find the value of x. (See Example 2.)
5.
6.
25
X
T 20
7
x
24
39
E
H
x
36
8.
F
15
16
G
12
24.
4
x
26.
x
3.5 ft
4.6 ft
23 ft
486
Chapter 9
2
16
8
27
5.8 ft
x
b
25.
z
12.8 ft
16
x
C
10
10..
26.3 ft
6
34
30
10
y
5
B
9.
R
23.
D
25
18
16
A
x
22.
x
Z
S
7.
16
21.
12
8
y
4
Q
W Y
5
Right Triangles and Trigonometry
x
ERROR ANALYSIS In Exercises 27 and 28, describe and
MATHEMATICAL CONNECTIONS In Exercises 31–34, find
correct the error in writing an equation for the given
diagram.
the value(s) of the variable(s).
27.
✗
x
✗
6
a+5
z
y
33.
v
z2 = w
18
34.
x
z
⋅(w + v)
y
16
12
y
32
24
x
z
e
g
d
h
35. REASONING Use the diagram. Decide which
f
proportions are true. Select all that apply.
C
D
⋅
d2 = f h
A
MODELING WITH MATHEMATICS In Exercises 29 and 30,
use the diagram. (See Example 5.)
B
A —=—
○
DC DB
DB
DA
B —=—
○
CB BD
CA
BA
D —=—
○
BC BA
C —=—
○
BA CA
BA
CB
DB
DA
36. ANALYZING RELATIONSHIPS You are designing
7.2 ft
5.5 ft
Ex. 29
b+3
8
32.
12
w
28.
31.
6 ft
9.5 ft
Ex. 30
a diamond-shaped kite. You know that
AD = 44.8 centimeters, DC = 72 centimeters, and
AC = 84.8 centimeters. You want to use a straight
—. About how long should it be? Explain
crossbar BD
your reasoning.
29. You want to determine the height of a monument at
D
a local park. You use a cardboard square to line up
the top and bottom of the monument, as shown at the
above left. Your friend measures the vertical distance
from the ground to your eye and the horizontal
distance from you to the monument. Approximate
the height of the monument.
30. Your classmate is standing on the other side of the
monument. She has a piece of rope staked at the
base of the monument. She extends the rope to the
cardboard square she is holding lined up to the top
and bottom of the monument. Use the information
in the diagram above to approximate the height of
the monument. Do you get the same answer as in
Exercise 29? Explain your reasoning.
A
B
C
37. ANALYZING RELATIONSHIPS Use the Geometric
Mean Theorems (Theorems 9.7 and 9.8) to find
AC and BD.
B
20
A
Section 9.3
15
D
C
Similar Right Triangles
487
38. HOW DO YOU SEE IT? In which of the following
40. MAKING AN ARGUMENT Your friend claims the
triangles does the Geometric Mean (Altitude)
Theorem (Theorem 9.7) apply?
A
○
geometric mean of 4 and 9 is 6, and then labels the
triangle, as shown. Is your
friend correct? Explain
9
4
6
your reasoning.
B
○
In Exercises 41 and 42, use the given statements to
prove the theorem.
C
○
Given △ABC is a right triangle.
— is drawn to hypotenuse AB
—.
Altitude CD
D
○
41. PROVING A THEOREM Prove the Geometric Mean
(Altitude) Theorem (Theorem 9.7) by showing that
CD2 = AD BD.
⋅
42. PROVING A THEOREM Prove the Geometric Mean
39. PROVING A THEOREM Use the diagram of △ABC.
(Leg) Theorem (Theorem 9.8) by showing that
CB2 = DB AB and AC2 = AD AB.
⋅
Copy and complete the proof of the Pythagorean
Theorem (Theorem 9.1).
43. CRITICAL THINKING Draw a right isosceles triangle
Given In △ABC, ∠BCA
is a right angle.
A f
Prove c2 = a2 + b2
b
D
1. In △ABC, ∠BCA is a
and label the two leg lengths x. Then draw the altitude
to the hypotenuse and label its length y. Now, use the
Right Triangle Similarity Theorem (Theorem 9.6)
to draw the three similar triangles from the image
and label any side length that is equal to either
x or y. What can you conclude about the relationship
between the two smaller triangles? Explain
your reasoning.
c
e
a
C
STATEMENTS
B
REASONS
1. ________________
right angle.
2. Draw a perpendicular
segment (altitude)
—.
from C to AB
44. THOUGHT PROVOKING The arithmetic mean and
2. Perpendicular
geometric mean of two nonnegative numbers x and y
are shown.
x+y
arithmetic mean = —
2
—
geometric mean = √ xy
Postulate
(Postulate 3.2)
3. ce = a2 and cf = b2
3. ________________
4. ce + b2 = ____ + b2
4. Addition Property
Write an inequality that relates these two means.
Justify your answer.
of Equality
5. ce + cf = a2 + b2
5. ________________
6. c(e + f ) = a2 + b2
6. ________________
7. e + f = ____
7. Segment Addition
8. c
9.
⋅
c2
45. PROVING A THEOREM Prove the Right Triangle
Similarity Theorem (Theorem 9.6) by proving three
similarity statements.
Postulate
(Postulate 1.2)
c = a2 + b2
= a2 + b2
⋅
Given △ABC is a right triangle.
— is drawn to hypotenuse AB
—.
Altitude CD
8. ________________
Prove △CBD ∼ △ABC, △ACD ∼ △ABC,
△CBD ∼ △ACD
9. Simplify.
Maintaining Mathematical Proficiency
Reviewing what you learned in previous grades and lessons
Solve the equation for x. (Skills Review Handbook)
x
5
46. 13 = —
488
Chapter 9
x
4
47. 29 = —
78
x
48. 9 = —
Right Triangles and Trigonometry
115
x
49. 30 = —
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