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Proving Triangle Congruence 5.6 Essential Question —
5.6
TEXAS ESSENTIAL
KNOWLEDGE AND SKILLS
Proving Triangle Congruence
by ASA and AAS
Essential Question
What information is sufficient to determine
whether two triangles are congruent?
G.5.A
G.6.B
Determining Whether SSA Is Sufficient
Work with a partner.
a. Use dynamic geometry software to construct △ABC. Construct the triangle so that
— has a length of 3 units, and BC
— has a length of 2 units.
vertex B is at the origin, AB
b. Construct a circle with a radius of 2 units centered at the origin. Locate point D
—. Draw BD
—.
where the circle intersects AC
Sample
3
A
D
2
1
C
0
−3
−2
−1
B
0
1
2
3
−1
−2
MAKING
MATHEMATICAL
ARGUMENTS
To be proficient in math,
you need to recognize and
use counterexamples.
Points
A(0, 3)
B(0, 0)
C(2, 0)
D(0.77, 1.85)
Segments
AB = 3
AC = 3.61
BC = 2
AD = 1.38
Angle
m∠A = 33.69°
c. △ABC and △ABD have two congruent sides and a nonincluded congruent angle.
Name them.
d. Is △ABC ≅ △ABD? Explain your reasoning.
e. Is SSA sufficient to determine whether two triangles are congruent? Explain
your reasoning.
Determining Valid Congruence Theorems
Work with a partner. Use dynamic geometry software to determine which of the
following are valid triangle congruence theorems. For those that are not valid, write
a counterexample. Explain your reasoning.
Possible Congruence Theorem
Valid or not valid?
SSS
SSA
SAS
AAS
ASA
AAA
Communicate Your Answer
3. What information is sufficient to determine whether two triangles are congruent?
4. Is it possible to show that two triangles are congruent using more than one
congruence theorem? If so, give an example.
Section 5.6
Proving Triangle Congruence by ASA and AAS
273
5.6 Lesson
What You Will Learn
Use the ASA and AAS Congruence Theorems.
Core Vocabul
Vocabulary
larry
Previous
congruent figures
rigid motion
Using the ASA and AAS Congruence Theorems
Theorem
Theorem 5.10 Angle-Side-Angle (ASA) Congruence Theorem
If two angles and the included side of one triangle are congruent to two angles and
the included side of a second triangle, then the two triangles are congruent.
— ≅ DF
—, and ∠C ≅ ∠F,
If ∠A ≅ ∠D, AC
then △ABC ≅ △DEF.
B
E
C
Proof p. 274
A
D
F
Angle-Side-Angle (ASA) Congruence Theorem
— ≅ DF
—, ∠C ≅ ∠F
Given ∠A ≅ ∠D, AC
Prove △ABC ≅ △DEF
B
E
C
A
D
F
First, translate △ABC so that point A maps to point D, as shown below.
B
C
A
E
B′
E
D
F
D
C′
F
This translation maps △ABC to △DB′C′. Next, rotate △DB′C′ counterclockwise
through ∠C′DF so that the image of ⃗
DC′ coincides with ⃗
DF, as shown below.
E
E
B′
D
D
C′
F
F
B″
— ≅ DF
—, the rotation maps point C′ to point F. So, this rotation maps
Because DC′
△DB′C′ to △DB″F. Now, reflect △DB″F in the line through points D and F, as
shown below.
E
E
D
F
D
F
B″
Because points D and F lie on ⃖⃗
DF, this reflection maps them onto themselves. Because
a reflection preserves angle measure and ∠B″DF ≅ ∠EDF, the reflection maps ⃗
DB″ to
⃗
DE. Similarly, because ∠B″FD ≅ ∠EFD, the reflection maps ⃗
FB″ to ⃗
FE. The image of
B″ lies on ⃗
DE and ⃗
FE. Because ⃗
DE and ⃗
FE only have point E in common, the image of
B″ must be E. So, this reflection maps △DB″F to △DEF.
Because you can map △ABC to △DEF using a composition of rigid motions,
△ABC ≅ △DEF.
274
Chapter 5
Congruent Triangles
Theorem
Theorem 5.11 Angle-Angle-Side (AAS) Congruence Theorem
If two angles and a non-included side of one triangle are congruent to two angles
and the corresponding non-included side of a second triangle, then the two
triangles are congruent.
If ∠A ≅ ∠D, ∠C ≅ ∠F,
— ≅ EF
—, then
and BC
△ABC ≅ △DEF.
E
B
A
C
D
F
Proof p. 275
Angle-Angle-Side (AAS) Congruence Theorem
Given ∠A ≅ ∠D,
∠C ≅ ∠F,
— ≅ EF
—
BC
Prove
B
△ABC ≅ △DEF
E
A
C
F
D
You are given ∠A ≅ ∠D and ∠C ≅ ∠F. By the Third Angles Theorem (Theorem 5.4),
— ≅ EF
—. So, two pairs of angles and their included sides
∠B ≅ ∠E. You are given BC
are congruent. By the ASA Congruence Theorem, △ABC ≅ △DEF.
Identifying Congruent Triangles
Can the triangles be proven congruent with the information given in the diagram?
If so, state the theorem you would use.
a.
b.
c.
COMMON ERROR
You need at least one pair
of congruent corresponding
sides to prove two triangles
are congruent.
SOLUTION
a. The vertical angles are congruent, so two pairs of angles and a pair of non-included
sides are congruent. The triangles are congruent by the AAS Congruence Theorem.
b. There is not enough information to prove the triangles are congruent, because no
sides are known to be congruent.
c. Two pairs of angles and their included sides are congruent. The triangles are
congruent by the ASA Congruence Theorem.
Monitoring Progress
Help in English and Spanish at BigIdeasMath.com
1. Can the triangles be proven congruent with
X
the information given in the diagram? If so,
state the theorem you would use.
4
W
Section 5.6
3
1
2
Y
Z
Proving Triangle Congruence by ASA and AAS
275
Copying a Triangle Using ASA
Construct a triangle that is congruent to △ABC using the
ASA Congruence Theorem. Use a compass and straightedge.
C
A
SOLUTION
Step 1
Step 2
Step 3
B
Step 4
F
D
E
D
Construct a side
— so that it is
Construct DE
—.
congruent to AB
E
D
Construct an angle
Construct ∠D with
⃗ so
vertex D and side DE
that it is congruent to ∠A.
E
D
Construct an angle
Construct ∠E with
⃗ so
vertex E and side ED
that it is congruent to ∠B.
E
Label a point
Label the intersection of
the sides of ∠D and ∠E
that you constructed in
Steps 2 and 3 as F. By the
ASA Congruence Theorem,
△ABC ≅ △DEF.
Using the ASA Congruence Theorem
A
Write a proof.
C
— EC
—, BD
— ≅ BC
—
Given AD
Prove △ABD ≅ △EBC
B
D
SOLUTION
STATEMENTS
REASONS
— —
1. AD EC
1. Given
A 2. ∠D ≅ ∠C
S
E
2. Alternate Interior Angles Theorem
(Thm. 3.2)
— ≅ BC
—
3. BD
3. Given
A 4. ∠ABD ≅ ∠EBC
4. Vertical Angles Congruence Theorem
(Thm 2.6)
5. △ABD ≅ △EBC
5. ASA Congruence Theorem
Monitoring Progress
Help in English and Spanish at BigIdeasMath.com
— ⊥ AD
—, DE
— ⊥ AD
—, and AC
— ≅ DC
—. Prove △ABC ≅ △DEC.
2. In the diagram, AB
E
A
B
276
Chapter 5
Congruent Triangles
C
D
Using the AAS Congruence Theorem
Write a proof.
— GK
—, ∠ F and ∠ K are right angles.
Given HF
F
G
H
K
Prove △HFG ≅ △GKH
SOLUTION
STATEMENTS
REASONS
— GK
—
1. HF
1. Given
A 2. ∠GHF ≅ ∠HGK
2. Alternate Interior Angles Theorem
(Theorem 3.2)
3. ∠ F and ∠ K are right angles.
A 4. ∠ F ≅ ∠ K
3. Given
4. Right Angles Congruence Theorem
(Theorem 2.3)
— ≅ GH
—
S 5. HG
5. Reflexive Property of Congruence
(Theorem 2.1)
6. △HFG ≅ ∠GKH
6. AAS Congruence Theorem
Monitoring Progress
Help in English and Spanish at BigIdeasMath.com
— ≅ VU
—. Prove △RST ≅ △VUT.
3. In the diagram, ∠S ≅ ∠U and RS
R
U
T
S
V
Concept Summary
Triangle Congruence Theorems
You have learned five methods for proving that triangles are congruent.
SAS
E
B
A
D
s only)
HL (right △
SSS
E
B
F
C
Two sides and the
included angle are
congruent.
A
D
ASA
E
F
C
B
A
All three sides are
congruent.
D
AAS
E
B
F
C
The hypotenuse and
one of the legs are
congruent.
A
D
E
B
F
C
Two angles and the
included side are
congruent.
A
D
F
C
Two angles and a
non-included side
are congruent.
In the Exercises, you will prove three additional theorems about the congruence of right triangles:
Hypotenuse-Angle, Leg-Leg, and Angle-Leg.
Section 5.6
Proving Triangle Congruence by ASA and AAS
277
Exercises
5.6
Dynamic Solutions available at BigIdeasMath.com
Vocabulary and Core Concept Check
1. WRITING How are the AAS Congruence Theorem (Theorem 5.11) and the ASA Congruence
Theorem (Theorem 5.10) similar? How are they different?
2. WRITING You know that a pair of triangles has two pairs of congruent corresponding angles.
What other information do you need to show that the triangles are congruent?
Monitoring Progress and Modeling with Mathematics
In Exercises 3–6, decide whether enough information
is given to prove that the triangles are congruent. If so,
state the theorem you would use. (See Example 1.)
3. △ABC, △QRS
4. △ABC, △DBC
— — — —
10. ∠C ≅ ∠F, AB ≅ DE , BC ≅ EF
— —
C
A
Q
11. ∠B ≅ ∠E, ∠C ≅ ∠F, AC ≅ DE
S
— —
12. ∠A ≅ ∠D, ∠B ≅ ∠E, BC ≅ EF
A
R
5. △XYZ, △JKL
Y
— —
9. ∠A ≅ ∠D, ∠C ≅ ∠F, AC ≅ DF
B
B
In Exercises 9–12, decide whether you can use the given
information to prove that △ABC ≅ △DEF. Explain
your reasoning.
C
D
6. △RSV, △UTV
R
K
S
CONSTRUCTION In Exercises 13 and 14, construct a
triangle that is congruent to the given triangle using
the ASA Congruence Theorem (Theorem 5.10). Use
a compass and straightedge.
13.
Z L
J
X
U
T
In Exercises 7 and 8, state the third congruence
statement that is needed to prove that △FGH ≅ △LMN
using the given theorem.
F
L
G
D
F
L
correct the error.
15.
✗
K
H
L
G
F
J
N
— ≅ MN
—, ∠G ≅ ∠M, ___ ≅ ____
7. Given GH
16.
✗
Q
X
W
Use the AAS Congruence Theorem (Thm. 5.11).
— ≅ LM
—, ∠G ≅ ∠M, ___ ≅ ____
8. Given FG
Use the ASA Congruence Theorem (Thm. 5.10).
278
Chapter 5
Congruent Triangles
K
ERROR ANALYSIS In Exercises 15 and 16, describe and
M
H
J
14.
E
V
R
S
V
△JKL ≅ △FHG
by the ASA
Congruence
Theorem.
△QRS ≅ △VWX
by the AAS
Congruence Theorem.
PROOF In Exercises 17 and 18, prove that the triangles
are congruent using the ASA Congruence Theorem
(Theorem 5.10). (See Example 2.)
17. Given
—.
M is the midpoint of NL
— ⊥ NQ
—, NL
— ⊥ MP
—, QM
— PL
—
NL
Q
and a leg of a right triangle are congruent to an angle
and a leg of a second right triangle, then the triangles
are congruent.
24. REASONING What additional information do
you need to prove △JKL ≅ △MNL by the ASA
Congruence Theorem (Theorem 5.10)?
△NQM ≅ △MPL
Prove
23. Angle-Leg (AL) Congruence Theorem If an angle
— ≅ KJ
—
A KM
○
P
— ≅ NH
—
B KH
○
N
L
M
L
H
C ∠M ≅ ∠J
○
N
D ∠LKJ ≅ ∠LNM
○
— —
M
K
J
18. Given AJ ≅ KC , ∠BJK ≅ ∠BKJ, ∠A ≅ ∠C
Prove
25. MATHEMATICAL CONNECTIONS This toy
△ABK ≅ △CBJ
contains △ABC and △DBC. Can you conclude that
△ABC ≅ △DBC from the given angle measures?
Explain.
B
A
J
K
C
C
A
D
B
PROOF In Exercises 19 and 20, prove that the triangles
are congruent using the AAS Congruence Theorem
(Theorem 5.11). (See Example 3.)
m∠ABC = (8x — 32)°
— ≅ UW
—, ∠X ≅ ∠Z
19. Given VW
Prove
m∠DBC = (4y — 24)°
m∠BCA = (5x + 10)°
△XWV ≅ △ZWU
Z
V
m∠BCD = (3y + 2)°
X
Y
m∠CAB = (2x — 8)°
U
m∠CDB = (y − 6)°
W
26. REASONING Which of the following congruence
20. Given
Prove
statements are true? Select all that apply.
∠NKM ≅ ∠LMK, ∠L ≅ ∠N
— ≅ UV
—
A TU
○
△NMK ≅ △LKM
L
B △STV ≅ △XVW
○
N
W
S
X
C △TVS ≅ △VWU
○
K
M
D △VST ≅ △VUW
○
T
U
V
PROOF In Exercises 21–23, write a paragraph proof for
27. PROVING A THEOREM Prove the Converse of the
the theorem about right triangles.
21. Hypotenuse-Angle (HA) Congruence Theorem
If an angle and the hypotenuse of a right triangle are
congruent to an angle and the hypotenuse of a second
right triangle, then the triangles are congruent.
22. Leg-Leg (LL) Congruence Theorem If the legs of
a right triangle are congruent to the legs of a second
right triangle, then the triangles are congruent.
Section 5.6
Base Angles Theorem (Theorem 5.7). (Hint: Draw
an auxiliary line inside the triangle.)
28. MAKING AN ARGUMENT Your friend claims to
be able to rewrite any proof that uses the AAS
Congruence Theorem (Thm. 5.11) as a proof that
uses the ASA Congruence Theorem (Thm. 5.10).
Is this possible? Explain your reasoning.
Proving Triangle Congruence by ASA and AAS
279
29. MODELING WITH MATHEMATICS When a light ray
31. CONSTRUCTION Construct a triangle. Show that there
from an object meets a mirror, it is reflected back to
your eye. For example, in the diagram, a light ray
from point C is reflected at point D and travels back
to point A. The law of reflection states that the angle
of incidence, ∠CDB, is congruent to the angle of
reflection, ∠ADB.
a. Prove that △ABD is
congruent to △CBD.
is no AAA congruence rule by constructing a second
triangle that has the same angle measures but is not
congruent.
32. THOUGHT PROVOKING Graph theory is a branch of
mathematics that studies vertices and the way they
are connected. In graph theory, two polygons are
isomorphic if there is a one-to-one mapping from one
polygon’s vertices to the other polygon’s vertices that
preserves adjacent vertices. In graph theory, are any
two triangles isomorphic? Explain your reasoning.
A
Given ∠CDB ≅ ∠ADB,
— ⊥ AC
—
DB
Prove △ABD ≅ △CBD
b. Verify that △ACD is
isosceles.
B
c. Does moving away from
the mirror have any effect
on the amount of his or
her reflection a person
sees? Explain.
33. MATHEMATICAL CONNECTIONS Six statements are
given about △TUV and △XYZ.
D
— ≅ XY
—
TU
— ≅ YZ
—
UV
— ≅ XZ
—
TV
∠T ≅ ∠X
∠U ≅ ∠Y
∠V ≅ ∠Z
U
C
T
30. HOW DO YOU SEE IT? Name as many pairs of
congruent triangles as you can from the diagram.
Explain how you know that each pair of triangles
is congruent.
P
Z
a. List all combinations of three given statements
that would provide enough information to prove
that △TUV is congruent to △XYZ.
b. You choose three statements at random. What is
the probability that the statements you choose
provide enough information to prove that the
triangles are congruent?
T
R
Maintaining Mathematical Proficiency
Reviewing what you learned in previous grades and lessons
Find the coordinates of the midpoint of the line segment with the given endpoints. (Section 1.3)
34. C(1, 0) and D(5, 4)
35. J(−2, 3) and K(4, −1)
Use a compass and straightedge to copy the angle. (Section 1.5)
37.
38.
A
280
Chapter 5
B
Congruent Triangles
X
Y
Q
S
V
36. R(−5, −7) and S(2, −4)
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