# Conditions for Congruence Day

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```					                           Conditions for Congruence
Pre-Algebra

Day 1

Warmup:        (Use Questions 1 and 2 to review vocabulary)
1. Draw an acute angle. How do you know it is an acute angle?
2. Draw an obtuse angle. How do you know it is an obtuse angle?
Now let them work on some for themselves.
3. For each of the following pictures, determine if it is: ‘bigger’ ‘smaller’ or ‘equal’
to 90°. Be prepared to defend your answer. (Note: #7 has 3 angles, and #8 has 2.)
Display OT1: Name that Angle
Closure: let students discuss answers. Remind them of the vocabulary – acute, obtuse, right.

Activity: Classifying triangles
Put students in groups of 2-3, and give each group a bag with a set of cut-out triangles in them, a
protractor and a centimeter ruler. (The questions are duplicated on OT2 for your convenience.)
1. Which of the shapes are obtuse triangles?
2. Which of the shapes are right triangles?
3. Which of the shapes are acute triangles?
4. Which of the shapes are equilateral triangles?
5. Which of the shapes are isosceles triangles?
6. Which of the shapes are scalene triangles?
7. Which of the shapes are EXACTLY the same triangle? (These are called congruent.)
How do you know that they are the same?
8. Why can’t these triangles be congruent?
a) C and H
b) E and I
c) G and I
d) A and G
9. How will you know when two triangles are congruent?
Activity: “Moves”, “Flips”, “Spins” and congruence
Repeat this part of the activity twice -- using shapes B and E the first time, and shapes F and J
the second time. (The questions are duplicated on OT3 for your convenience.)
1. Place the two triangles directly on top of each other. (Call this the HOME position.)
How can that help to convince you that they are congruent?
2. Move the top triangle over to the right and up until it no longer touches the bottom one.
a. Sketch a drawing of the two triangles.
b. Are the two triangles congruent?
3. Take the triangle you moved and flip it over along one of its sides.
a) Sketch a drawing of the two triangles now.
b) Are the two triangles congruent?
4. Now take that same triangle and spin it around one of its corners just a little bit.
a) Sketch a drawing of the two triangles now..
b) Are the two triangles congruent?
a) Will moving one affect whether they are congruent? (translation)
b) Will flipping one affect whether they are congruent? (reflection)
c) Will spinning one affect whether they are congruent? (rotation)

Activity: Pair Up Equal Parts
The given triangles are congruent; students need to name vertex points, angles and sides that
correspond. Use OT4: Pair Up Equal Parts

Homework: Congruence I
Name that Angle
ACUTE, RIGHT, or OBTUSE

#1.               #2.

#3.               #4.

#5.               #6.

#7.               #8.
A

G
H
D

E               B

I
J

C               F
Classifying triangles
1. Which of the shapes are obtuse triangles?
2. Which of the shapes are right triangles?
3. Which of the shapes are acute triangles?
4. Which of the shapes are equilateral triangles?
5. Which of the shapes are isosceles triangles?
6. Which of the shapes are scalene triangles?
7. Which of the shapes are EXACTLY the
same triangle? (These are called congruent.)
How do you know that they are the same?
8. Why can’t these triangles be congruent?
a) C and H
b) E and I
c) G and I
d) A and G
9. How will you know when two triangles are
congruent?
“Moves”, “Flips”, “Spins”
and Congruence

1. Place the triangles directly on top of each other. How
can that help to convince you that they are congruent?
2. Move the top triangle over to the right and up until it no
longer touches the bottom one.
a) Sketch a drawing of the two triangles.
b) Are the two triangles congruent?
3. Take the triangle you moved, and flip it over along one
of its sides.
a) Sketch a drawing of the two triangles now.
b) Are the two triangles congruent?
4. Now take that same triangle and spin it around one of
its corners just a little bit.
a) Sketch a drawing of the two triangles now.
b) Are the two triangles congruent?
a) does moving one affect whether they are congruent?
(translation)
b) does flipping one affect whether they are congruent?
(reflection)
c) does spinning one affect whether they are congruent?
(rotation)
Pair Up Equal Parts
Directions: The two triangles are known to be congruent.
Determine the vertex points, angles and sides that match
up.
A

C                      B

D
A

E

C                         D
B

A            D

C                       B
Congruence I

1.

2.

Hint: What do you know about the angles in a triangle?

3.
Conditions for Congruence
Pre-Algebra

Day 2
Warmup -- Use OT1:        Congruence and Area
1. A rectangle is formed by putting two congruent squares adjacent to each other. Then
a triangle is formed by drawing a line from one corner to the opposite side, and then
to the other corner as shown. Explain why the triangle must have an area which is
one half the area of the rectangle.
Provide a hint by drawing an auxiliary line to form an altitude for the triangle.
2. “If two triangles have the same area, then they are congruent.” Is this a true
statement? If it is, provide an argument for why it must be true. If it is not, then find

Activity: Conditions for Congruence
Purpose: Students use paper strips, brads and cut-out angle measures to investigate the
sufficient (SSS, SAS, ASA, AAS) and insufficient (SSA and AAA) conditions for
proving triangles congruent.

Materials (per group): 6 paper strips of different lengths (out of 3 possible) made from
computer paper edges
6 angle cut-outs (out of 3 possible) made from transparency acetate

Introduction – Use OT2: Building Congruent Triangles
Use this to reinforce the instructional outcome of yesterday’s lesson – that when two
triangles are congruent, the corresponding parts match. Notice that it is stated as part of a
definition, and that it is technically the converse statement: when all corresponding
parts match, the two triangles are congruent.
Activity Directions – Use OT3: “Conditions for Congruence” Activity
Two triangles are defined to be congruent if the three pairs of corresponding
angles and the three pairs of corresponding sides are congruent. However, there
are several ways to construct congruent triangles using fewer than the six parts.
As a group, try to use the least amount of information to create a pair of congruent
triangles with the material given. Keep marked drawings of your findings and
what information was needed. Also keep track of methods which do not produce
congruent triangles for a class discussion.
Clarifying Directions – Use OT4: Sample student solution
Use this if you see students need more specific directions about how to record the
examples that they find.

Class Discussion – Use OT5: Discussion Questions
•   Is it possible to build congruent triangles when only 1 piece of information is
identified? (one side or one angle) Explain.
•   Is it possible to build congruent triangles when only 2 pieces of information are
identified? (two sides or two angles or one side and one angle) Explain.
•   Is it possible to build congruent triangles when only 3 pieces of information are
identified?
o For what kinds of situations did this work? (SSS, SAS, ASA, AAS)
o For what kinds of situations did this not work? (SSA, AAA)
•   Look at this drawing: how does it help
convince us that SSS is enough information
to conclude that two triangles are congruent?
S2                   S3
(Note: Figure provided on OT6)

S1

•   Look at this triangle that is only partly finished.
All that is left to do is put the other side on it
(the angles will be determined then.)                       S2
If another triangle started out the same way
(same two lengths, same angle), would IT be                 A1
congruent to this triangle when the third side                        S1
was put on IT? Explain.
(Note: Figure provided on OT6)

•   Look at this triangle that is only partly finished.
All that is left to do is extend the two remaining
arms until a triangle is formed.
A1              A2
If another triangle started out the same way
S1
(same two angles, same length), would IT be
congruent to this triangle when the third side
was put on IT? Explain.
(Note: Figure provided on OT6)
•   You may have found that two angles and a
side which is not between them is enough
information to build congruent triangles (AAS).            5
Suppose you have a situation like the one shown
30°             40°
where the side has length 5, and the angles have
measures 30° and 40°.
(Note: Figure provided on OT7)
o What will the third angle have to be? Why?
o Once you know that angle, this AAS situation should look like one of the other
ones we just discussed. Will it be like SSS, SAS, or ASA? How come?

•   You probably found that knowing two sides and an angle which is not between them
is not enough information to build congruent triangles. Here is one way to explain
that:

S1
S2

A1

(Note: Figure provided on OT7)

You can extend the part of angle A1 in the direction that it must go, but you don’t
know how far to go. You can swing the side S2 around, since you don’t know another
angle. What does the picture suggest the problem is with a SSA approach to showing
two triangles are congruent?

•   Why is it that specifying all three angles
can never provide enough information
to show that two triangles are congruent?

(Note: Figure provided on OT7)

Homework: Congruence II
Congruence and Area

1. A rectangle is formed by putting two congruent
squares adjacent to each other. Then a triangle is
formed by drawing a line from one vertex to the
opposite side, then to the other vertex.

Explain why the triangle must have an area
which is one-half the area of the rectangle.

2. “If two triangles have the same area,
then they are congruent.”
Is this a true statement?
• If it is, provide an argument for why it
must be true.
• If it is not, then find an example to
Building Congruent Triangles

Two triangles are congruent if all 3 pairs
of corresponding angles and all 3 pairs of
corresponding sides are congruent.
A1        A1         A2    A2        A3    A3

A1        A1         A2    A2        A3    A3

A1        A1         A2    A2        A3    A3

S1   S1   S1

S2    S2   S2

S3   S3   S3        S3     S3   S3

S1   S1   S1
S2
S2
S2
“Conditions for Congruence”
Activity

There are several ways to construct congruent
triangles using fewer than the six parts.
As a group, try to use the least amount of
information to create a pair of congruent
triangles with the material given.
Keep marked drawings of your findings and
what information was needed. Also keep track
of methods which do not produce congruent
triangles for a class discussion.
Sample Student Solution

Using 2 pairs of congruent paper strips and
1 pair of congruent angles, we created a pair
of congruent triangles. We call this method
“Side-Angle-Side” Triangle Congruence.
Discussion Questions

• Is it possible to build congruent triangles when
only 1 piece of information is identified? (one side
or one angle) Explain.

• Is it possible to build congruent triangles when
only 2 pieces of information are identified?
(two sides or two angles or one side and one angle)
Explain.

• Is it possible to build congruent triangles when
only 3 pieces of information are identified?
o For what kinds of situations did this work?
o For what kinds of situations did this not work?
S2              S3

S1

S2

A1
S1

A1            A2
S1
5

30°          40°

S1
S2

A1

A3

A2            A1
Congruence II
Conditions for Congruence
Pre-Algebra

Day 3
Warmup -- Use OT1:       Congruence Notation and Vocabulary
When we know that two triangles are congruent, it is important to identify
corresponding parts – that is, the ones that match up with each other.
• We say: “Triangle ABC is congruent to triangle TUV”
• We write: ∆ABC ≅ ∆TUV
• That means the following parts are corresponding:
AB ≅ TU                   AC ≅ TV                BC ≅ UV
∠ABC ≅ ∠TUV              ∠ACB ≅ ∠TVU             ∠BAC ≅ ∠UTV
• Notice the markings that indicate which sides (or angles) match up

Now, answer the following questions: Use OT2: Congruence Relations
1) Suppose ∆CAT      ∆DOG. Find 3 pairs of sides and 3 pairs of angles that are also
congruent.
2) Suppose that ∆ABC ∆XYZ, m         A = 110°, and m    C = 35°. Find four pairs of
congruent angles.
Note: Be sure to point out the notation in Question #2 about “the measure of an angle”

Activity: Conditions for Congruence (cont.)
If necessary, finish the follow-up discussion from yesterday’s class.

Activity: Hey, Cableman!
• Form student groups; provide students with rulers and a copy of the handout.
• Let students read the directions, and solve the problem using measurement methods.
• When students have determined the solution reasonably well, display the
OT3: Cableman’s Boss
Ask students to check their solution using this method and explain how that drawing
can help solve the problem faster, and more accurately. Then ask them to identify the
geometric ideas that are being used in the drawing.

Homework: Congruence III
Congruence Notation
and Vocabulary

You are told that ∆ABC ≅ ∆TUV
U

V                    T

B

A                        C
Congruence Relations

1) Suppose ∆CAT ≅ ∆DOG. Find 3 pairs of
sides and 3 pairs of angles that are also
congruent.

2) Suppose that ∆ABC ≅ ∆XYZ, m∠A = 110°,
and m∠C = 35°. Find four pairs of congruent
angles.
Hey, Cableman!

Two houses are to be hooked up to cable TV by running service lines from
the houses to the same point where they will both connect to the main cable. The
main cable runs underground along the edge of the street in front of the houses.
By measuring, determine where the connection should be made to use the least
amount of cable altogether for the two service lines.

House
A
House
B

Main Cable
Cableman’s Boss

House
A
House
B

P

M
Congruence III
Directions: All of the following drawings show two congruent triangles. In each case, identify
which argument can be used to conclude that they are congruent:
SSS (side, side, side)      SAS (two sides and the angle in between them)
ASA (two angles and the side in between them)
Also, write the statement which says that the two triangles are congruent using mathematical
notation.

1.                                                  2.         C       Y                Z

A                  B       X

A

3.                                                  4.

C                           B
D

5.                                                  6.

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