1. Proving Triangles Congruent
Geometric Proofs
Proving
Triangles
Congruent

2. LET’S GET STARTED
Before we begin, let’s see how much you already know.
In your print materials there is a Entry-Test.
Complete it now.

3. CHECK YOURSELF SECTION 3 1. Congruence SECTION 1 1. AC  AB 2. C  B
2. Perpendicular
3. Equal
4. Parallel
5. Angle
6. Triangle
7. Line Segment AB
8. Measure of Angle
SECTION 1
1. AC  AB
2. C  B
3. Isosceles Triangle
SECTION 2
1. AB  CD
2. AB  BE
3. Right Triangle

4. HOW DID YOU DO? Excellent - 12 - 14 correct Great - 10 -12 correct
Good correct
If you fall into any of these categories…
continue to next page.

5. What do you need to know in order to complete a proof?
Apply Geometric Marking Symbols
Identify Geometric Postulates, Definitions, and Theorems.
Identify Two-Column Proof Method.

6. How do you mark a figure? Angles- using arcs on each angle.
example:1  2 A
Segments- using slash marks on each segment.
example: AB  AC 1 2 B C

7. Parallel Lines – using an arrow on each line.
example: AD || BC A D
Perpendicular lines – using a right angle box.
example: AB  BC B C

8. What Postulates and Theorems are used to prove Triangles Congruent?
SSS Postulate - If the sides of one triangle are congruent to the sides of another triangle, then the triangles are congruent.
SAS Postulate - If two sides and the included angle of one triangle are congruent to two sides and the included angle of another triangle, then the triangles are congruent.
SSS
SAS

9. ASA Postulate - If two angles and the included side of one triangle are congruent to two angles and the included side of another triangle, then the triangles are congruent.
AAS Theorem - If two angles and a non included side are congruent to the corresponding two angles and side of another triangle, then the triangles are congruent.
ASA
AAS

10. What is the Two-Column Proof Method?
Let’s take the following paragraph proof and transform it into a two-column proof….

11. Two – Column Proof Statements Justifications D A E B C
Given: E is the midpoint of segment AC and segment BD
Prove: ABE  CED.
Statements
Justifications D A
1. E is the midpoint of AC and BD
1. Given
2. AE  EC and BE  ED
2. Midpoint Theorem E B C
. Paragraph Proof
Since E is the midpoint of segment AC, segment AE is congruent to EC by midpoint theorem. Since E is the midpoint of segment BD, segment BE is congruent to segment ED by midpoint theorem. Angle AEB and angle CED are vertical angles by definition. Therefore angle AEB is congruent to angle CED because all vertical angles are congruent. Triangle ABE is congruent to triangle CED by the side-angle-side postulate.
3. AEB and CED are vertical angles.
3. Definition of Vertical Angles
4. AEBCED
4. All Vertical angles are congruent.
5. ABE  CED
5. SAS Postulate.

12. What Have We Learned So Far?
The symbols used to mark figures.
Arcs, Slashes, Arrows, and Boxes
The Postulates and Theorems used to prove triangles are congruent.
SSS, SAS, ASA, and AAS
What a Two-Column proof looks like.
Column 1 is mathematical statements. Column 2 is justifications of those statements.

13. Assessment Time 
In your print materials there is a Unit 1 Assessment.
Stop and Complete it now.

14. Check Yourself Section 1 Section 2 A D 1 3 B 4 2 C Section 3
Midpoint Theorem.
All Vertical Angles are Congruent
SSS Postulate
SAS Postulate
ASA Postulate
Angle Bisector Theorem
Segment Bisector Theorem
Corresponding Angles Theorem
Section 1 A D 1 3 B 4 2 C
Statements
Justifications
Section 3
1. M is midpoint of AB
1. Given
2. AM = MB
2. Defn. of midpoint
3. AM  MB
3. Midpoint Theorem.

15. HOW DID YOU DO? Excellent - 12 - 15 correct Great - 10 -12 correct
Good correct
If you fall into any of these categories…
continue to next page.
If not, click here…

16. What are the first steps in a proof?
Read and understand the problem.
Analyze the given information by…
Locate and label the diagram with the given information.
Determine the relationship between the given, prove, and diagram

17. Read and Understand the Problem
Example:
Given: 1 &2 are rt. And ST  TP.
Prove: STR  PTR
1. Re-state the given statement.
Angle one and angle two are right angles. Segment ST is congruent to segment TP. S
2. What is supposed to be proved? 1 3 T R
Triangle STR is congruent to triangle PTR. 2 4 P

18. Analyze the Given Information
Example:
Given: 1 &2 are rt. And ST  TP.
Prove: STR  PTR
1. Mark the diagram with the given information.
2. Determine the relationship between the given, prove, and diagram.
Angle 1 and angle 2 are congruent because all right angles are congruent. Segment TR is congruent to itself. S 1 3 T R 2 4 P

19. Let’s Review The first two steps to solve a proof are…….
Read and Understand the problem.
Analyze the given information by marking the diagram and determining the relationship between the statements and the diagram.

20. Assessment Time 
In your print materials there is a Unit 2 Assessment.
Stop and Complete it now.

21. CHECK YOURSELF 1. SECTION 1
1. Segment EF is congruent to segment GH and segment EH is congruent to GF.
2. Triangle EFH is congruent to triangle GHF.
Angles YPH and HPX are right angles and they are congruent. Segment HP is congruent to itself. 2.
Segments AE and ED are congruent. Angles AEB and CED are vertical and congruent.

22. HOW DID YOU DO? Excellent - 4 correct Great – 3 correct
Good – 2 correct
If you fall into any of these categories…
continue to next page.
If not click here…

23. What are next steps in a proof?
Draw and Label Columns
Enter the Given statement as number 1 in both columns

24. Draw and Label Columns Example: Given: 1 &2 are rt. And ST  TP.
Prove: STR  PTR
Statements
Justifications S 1 3 T R 2 4 P

25. Enter the Given as #1 Example: Given: 1 &2 are rt. And ST  TP.
Prove: STR  PTR
Statements
Justifications
1. 1 &2 are rt. & ST  TP.
1. Given S 1 3 T R 2 4 P

26. Let’s Review The first four steps to solve a proof are…….
Read and Understand the problem.
Analyze the given information.
Draw and Label Columns.
Enter Given Statement.

27. Assessment Time 
In your print materials there is a Unit 3 Assessment.
Stop and Complete it now.

28. CHECK YOURSELF 1. SECTION 2 SECTION 1 1. Statements Justifications 2.
1. AB & 12
1. Given
Statements
Justifications
Statements
Justifications 2.
1. AB bisects DC & ABDC
1. Given

29. HOW DID YOU DO? Excellent - 3 correct Great – 2 correct
Good – 1 correct
If you fall into any of these categories…
continue to next page.
If not click here…

30. What are next steps in a proof?
Determine what can be assumed from the diagram and the theorem or postulate that allows the assumption.
Enter next step into chart.

31. Determine Assumptions
Remember the previous relationship step.
Angle 1 and angle 2 are congruent because all right angles are congruent. Segment TR is congruent to itself.
These are the assumptions!
Re-write them with symbols and justifications.
12: all right’s are .
TRTR: Reflexive Property()
Example:
Given: 1 &2 are rt. And ST  TP.
Prove: STR  PTR S 1 3 T R 2 4 P

32. Enter Assumptions into Chart
Example:
Given: 1 &2 are rt. And ST  TP.
Prove: STR  PTR
Statements
Justifications
1. 1 &2 are rt. & ST  TP.
2. 12
3. TRTR
Given
All Rt. ’s are .
Reflexive Prop.() S 1 3 T R 2 4 P

33. Let’s Review The first six steps to solve a proof are…….
Read and Understand the problem.
Analyze the given information.
Draw and Label Columns.
Enter Given Statement.
Determine Assumptions.
Enter Assumptions into chart.

34. Assessment Time 
In your print materials there is a Unit 4 Assessment.
Stop and Complete it now.

35. CHECK YOURSELF 1. SECTION 2 SECTION 1
Angles two and four are vertical angles by definition. They are also congruent because all vertical angles are congruent.
Segments MN and NP are congruent by definition of bisector. Segment NO is congruent to itself by reflexive property of equality.
Statements
Justifications
12
2&4 are vertical.
24
Given
Defn. of vert. ’s
All vert. ’s are . 2.
Statements
Justifications
MO  PO and MO bisects MP
MN  NP
NO  No
Given
Defn. of Bisector
Reflexive prop()

36. HOW DID YOU DO? Excellent – 4 correct Great – 3 correct
Good – 2 correct
If you fall into any of these categories…
continue to next page.
If not click here…

37. What are next steps in a proof?
Ask yourself “Is the last step listed the prove statement?”
If the answer is yes, then you are finished.
If the answer is no, then Determine the next assumption from the present information and enter it into the chart.

38. Is The Last Statement the Prove?
Example:
Given: 1 &2 are rt. And ST  TP.
Prove: STR  PTR
Statements
Justifications
1. 1 &2 are rt. & ST  TP.
2. 12
3. TRTR
Given
All Rt. ’s are .
Reflexive Prop.() S 1 3
No, What assumption could be made next? T R 2 4
By looking at the diagram, I see that the triangles are congruent by the side-angle-side postulate. P

39. Enter Assumptions into Chart
Example:
Given: 1 &2 are rt. And ST  TP.
Prove: STR  PTR
Statements
Justifications
1. Given
2. All Rt. ’s are .
3. Reflexive Prop.()
4. SAS Postulate
1. 1 &2 are rt. & ST  TP.
2. 12
3. TRTR
4.STRPTR S 1 3 T R 2 4
Now, the proof is complete since the last statement is the prove  YEAH P

40. Let’s Review All of the steps to solve a proof are…….
Read and Understand the problem.
Analyze the given information.
Draw and Label Columns.
Enter Given Statement.
Determine Assumptions.
Enter Assumptions into chart.
“Is the last statement the prove?” If not return to step 5.
Stay here to complete your final assessment in your print materials.
This way you may refer to the steps.
Good Luck 

41. CHECK YOURSELF SECTION 1 2. 1. Statements Justifications RLDC & LCRD
DL  DL
MGK RGK
Given
Reflexive prop.()
SSS Postulate.
Statements
Justifications
GK  MR & GK bisects MR.
GK  GK
MK  KR
GKM & GKR are rt.
GKM  GKR
MGKRGK
Given
Reflexive Prop().
Defn. of bisect.
Defn. of perpendicular.
All rt. Angles are .
SAS postulate.

42. You have officially completed this module on proofs!!!!
CONGRATULATIONS
You have officially completed this module on proofs!!!!