Download Notes: Proving Triangles Congruent

G.6
Proving
Triangles
Congruent
1
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2
The Idea of Congruence
Two geometric figures with
exactly the same size and
shape.
F
B
A
C
E
D
3
How much do you
need to know. . .
. . . about two triangles
to prove that they
are congruent?
4
Corresponding Parts
Previously we learned that if all six
pairs of corresponding parts (sides and
angles) are congruent, then the
triangles are congruent.
1. AB  DE
2. BC  EF
3. AC  DF
4.  A   D
5.  B   E
6.  C   F
ABC   DEF
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Do you need all six ?
NO !
SSS
SAS
ASA
AAS
HL
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Side-Side-Side (SSS)
If the sides of one triangle are congruent to the sides of a
second triangle, then the triangles are congruent.
Side
Side
1. AB  DE
2. BC  EF
3. AC  DF
Side
ABC   DEF
The triangles
are congruent by
SSS.
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Included Angle
The angle between two sides
 HGI
 G
 GIH
 I
 GHI
 H
This combo is called
side-angle-side, or just SAS.
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Included Angle
Name the included angle:
E
Y
S
YE and ES
 YES or E
ES and YS
 YSE or S
YS and YE
 EYS or Y
The other two
angles are the
NON-INCLUDED
angles.
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Side-Angle-Side (SAS)
If two sides and the included angle of one triangle are
congruent to the two sides and the included angle of another
triangle, then the triangles are congruent.
included
angle
Side
Side
1. AB  DE
2. A   D
3. AC  DF
Angle
ABC   DEF
The triangles
are congruent by
SAS.
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Included Side
The side between two angles
GI
HI
GH
This combo is called
angle-side-angle, or just ASA.
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Included Side
Name the included side:
E
Y
S
Y and E
YE
E and S
ES
S and Y
SY
The other two
sides are the
NON-INCLUDED
sides.
Angle-Side-Angle (ASA)
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If two angles and the included side of one triangle are
congruent to the two angles and the included side of another
triangle, then the triangles are congruent.
included
side
1. A   D
2. AB  DE
3.  B   E
Angle
Side
Angle
ABC   DEF
The triangles
are congruent by
ASA.
Angle-Angle-Side (AAS)
13
If two angles and a non-included side of one triangle are
congruent to the corresponding angles and side of another
triangle, then the triangles are congruent.
Non-included
side
1. A   D
2.  B   E
3. BC  EF
Angle
Side
Angle
ABC   DEF
The triangles
are congruent by
AAS.
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Warning: No SSA Postulate
There is no such
thing as an SSA
postulate!
Side
Angle
Side
The triangles are
NOTcongruent!
15
Warning: No SSA Postulate
There is no such
thing as an SSA
postulate!
NOT CONGRUENT!
BUT: SSA DOES work in one
situation!
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If we know that
the two triangles
are right
triangles!
Side
Side
Side
Angle
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We call this
HL,
for “Hypotenuse – Leg”
Hypotenuse
Hypotenuse
Leg
RIGHT Triangles!
These triangles ARE CONGRUENT by HL!
Remember!
The
triangles
must be
RIGHT!
Hypotenuse-Leg (HL)
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If the hypotenuse and a leg of a right triangle are congruent
to the hypotenuse and a leg of another right triangle, then the
triangles are congruent.
Right Triangle
Leg
1.AB  HL
2.CB  GL
3.C and G
are rt.  ‘s
ABC   DEF
The triangles
are congruent
by HL.
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Warning: No AAA Postulate
There is no such
thing as an AAA
postulate!
Same
Shapes!
E
B
A
C
D
NOT CONGRUENT!
Different
Sizes!
F
Congruence Postulates
and Theorems
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• SSS
• SAS
• ASA
• AAS
• AAA?
• SSA?
• HL
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Name That Postulate
(when possible)
SAS
SSA
Not enough
info!
ASA
AAS
22
Name That Postulate
(when possible)
AAA
SSA
Not enough
info!
Not enough
info!
SSS
SSA
HL
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Name That Postulate
(when possible)
Not enough
info!
Not enough
info!
SSA
SSA
HL
Not enough
info!
AAA
Vertical Angles,
Reflexive Sides and Angles
24
When two triangles touch, there may be
additional congruent parts.
Vertical Angles
Reflexive Side
side shared by two
triangles
25
Name That Postulate
(when possible)
Reflexive
Property
SAS
Vertical
Angles
AAS
Vertical
Angles
SAS
Reflexive
Property
SSA
Not enough
info!
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Reflexive Sides and Angles
When two triangles overlap, there may be
additional congruent parts.
Reflexive Side
side shared by two
triangles
Reflexive Angle
angle shared by two
triangles
Let’s Practice
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Indicate the additional information needed
to enable us to apply the specified
congruence postulate.
For ASA:
B  D
For SAS:
AC  FE
For AAS:
A  F
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What’s Next
Try Some Proofs
End Slide Show
Choose a
Problem.
Problem #1
SSS
Problem #2
SAS
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End Slide Show
B
A
C
D
A
C
B
E
D
X
Problem #3
ASA
W
Y
Z
Problem #4
Given: A  C
BE  BD
Prove: ABE  CBD
AAS
A
C
B
E
Statements
D
Reasons
Given
Vertical Angles Thm
Given
4. ABE  CBD AAS Postulate
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Problem #5
HL
A
Given ABC, ADC right s,
AB  AD
Prove:
3. AC  AC
ABC  ADC
B
Statements
1. ABC, ADC right s
AB  AD
D
C
Reasons
Given
Given
Reflexive Property
4. ABC  ADC
HL Postulate
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58
Congruence Proofs
1. Mark the Given.
2. Mark …
Reflexive Sides or Angles / Vertical Angles
Also: mark info implied by given info.
3. Choose a Method. (SSS , SAS, ASA)
4. List the Parts …
in the order of the method.
5. Fill in the Reasons …
why you marked the parts.
6. Is there more?
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Given implies Congruent Parts
midpoint


parallel
segment bisector
segments

angles
segments
angle bisector

angles
perpendicular

angles
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Example Problem
Given: AC bisects BAD
AB  AD
Prove: ABC  ADC
A
B
C
D
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… and
what it
implies
Step 1: Mark the Given
Given: AC bisects BAD
AB  AD
Prove: ABC  ADC
A
B
C
D
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Step 2: Mark . . .
Given: AC bisects BAD
AB  AD
Prove: ABC  ADC
•Reflexive Sides
•Vertical Angles
A
B
C
D
… if they exist.
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Step 3: Choose a Method
Given: AC bisects BAD
AB  AD
Prove: ABC  ADC
SSS
SAS
ASA
AAS
HL
B
A
C
D
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Step
4:
List
the
Parts
Given: AC bisects BAD
AB  AD
Prove: ABC  ADC
STATEMENTS
S
AB  AD
A
S
BAC  DAC
A
B
C
D
REASONS
AC  AC
… in the order of the Method
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Step 5: Fill in the Reasons
Given: AC bisects BAD
AB  AD
Prove: ABC  ADC
STATEMENTS
B
A
C
REASONS
S
AB  AD
Given
A
S
BAC  DAC
Def. of Bisector
Reflexive (prop.)
AC  AC
D
(Why did you mark those parts?)
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Step 6: Is there more?
Given: AC bisects BAD
AB  AD
Prove: ABC  ADC
STATEMENTS
S 1. AB  AD
1.
2. AC bisects BAD 2.
A 3. BAC  DAC 3.
4.
S 4. AC  AC
5. ABC  ADC 5.
A
B
C
REASONS
Given
Given
Def. of Bisector
Reflexive (prop.)
SAS (pos.)
D
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Congruent Triangles Proofs
1. Mark the Given and what it implies.
2. Mark … Reflexive Sides / Vertical Angles
3. Choose a Method. (SSS , SAS, ASA)
4. List the Parts …
in the order of the method.
5. Fill in the Reasons …
why you marked the parts.
6. Is there more?
73
Using CPCTC in Proofs

According to the definition of congruence, if two
triangles are congruent, their corresponding parts (sides
and angles) are also congruent.

This means that two sides or angles that are not marked
as congruent can be proven to be congruent if they are
part of two congruent triangles.

This reasoning, when used to prove congruence, is
abbreviated CPCTC, which stands for Corresponding
Parts of Congruent Triangles are Congruent.
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Corresponding Parts of
Congruent Triangles

For example, can you prove that sides AD and BC are
congruent in the figure at right?

The sides will be congruent if triangle ADM is congruent
to triangle BCM.





Angles A and B are congruent because they are marked.
Sides MA and MB are congruent because they are
marked.
Angles 1 and 2 are congruent because they are vertical
angles.
So triangle ADM is congruent to triangle BCM by ASA.
This means sides AD and BC are congruent by CPCTC.
Corresponding Parts of
Congruent Triangles
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A
two column proof that sides AD and BC
are congruent in the figure at right is shown
below:
Statement
Reason
MA  MB
Given
A  B
Given
1  2
Vertical angles
ADM  BCM
ASA
AD  BC
CPCTC
Corresponding Parts of
Congruent Triangles
76
A
two column proof that sides AD and BC
are congruent in the figure at right is shown
below:
Statement
Reason
MA  MB
Given
A  B
Given
1  2
Vertical angles
ADM  BCM
ASA
AD  BC
CPCTC
Corresponding Parts of
Congruent Triangles
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 Sometimes
it is necessary to add an auxiliary
line in order to complete a proof
 For example, to prove ÐR @ ÐO in this picture
Statement
Reason
FR @ FO
Given
RU @ OU
Given
UF @ UF
reflexive prop.
DFRU @ DFOU SSS
ÐR @ ÐO
CPCTC
Corresponding Parts of
Congruent Triangles
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 Sometimes
it is necessary to add an auxiliary
line in order to complete a proof
 For example, to prove ÐR @ ÐO in this picture
Statement
Reason
FR @ FO
Given
RU @ OU
Given
UF @ UF
Same segment
DFRU @ DFOU SSS
ÐR @ ÐO
CPCTC