1. Using Congruent Triangles Geometry Mrs. Spitz Fall 2004

2. Objectives: Use congruent triangles to plan and write proofs. Use congruent triangles to prove constructions are valid.

3. Planning a proof Knowing that all pairs of corresponding parts of congruent triangles are congruent can help you reach conclusions about congruent figures.

4. Planning a proof Suppose you want to prove that  PQS ≅  RQS in the diagram shown at the right. One way to do this is to show that ∆PQS ≅ ∆RQS by the SSS Congruence Postulate. Then, you can use the fact that corresponding parts of congruent triangles are congruent to conclude that  PQS ≅  RQS.

5. Ex. 1: Planning & Writing a Proof Given: AB ║ CD, BC ║ DA Prove: AB ≅CD Plan for proof: Show that ∆ABD ≅ ∆CDB. Then use the fact that corresponding parts of congruent triangles are congruent.

6. Ex. 1: Planning & Writing a Proof Solution: First copy the diagram and mark it with the given information. Then mark any additional information you can deduce. Because AB and CD are parallel segments intersected by a transversal, and BC and DA are parallel segments intersected by a transversal, you can deduce that two pairs of alternate interior angles are congruent.

7. Ex. 1: Paragraph Proof Because AD ║CD, it follows from the Alternate Interior Angles Theorem that  ABD ≅  CDB. For the same reason,  ADB ≅  CBD because BC ║DA. By the Reflexive property of Congruence, BD ≅ BD. You can use the ASA Congruence Postulate to conclude that ∆ABD ≅ ∆CDB. Finally because corresponding parts of congruent triangles are congruent, it follows that AB ≅ CD.

8. Ex. 2: Planning & Writing a Proof Given: A is the midpoint of MT, A is the midpoint of SR. Prove: MS ║TR. Plan for proof: Prove that ∆MAS ≅ ∆TAR. Then use the fact that corresponding parts of congruent triangles are congruent to show that  M ≅  T. Because these angles are formed by two segments intersected by a transversal, you can conclude that MS ║ TR.

9. Given: A is the midpoint of MT, A is the midpoint of SR. Prove: MS ║TR. Statements: 1.A is the midpoint of MT, A is the midpoint of SR. 2.MA ≅ TA, SA ≅ RA 3.  MAS ≅  TAR 4.∆MAS ≅ ∆TAR 5.  M ≅  T 6.MS ║ TR Reasons: 1.Given

10. Given: A is the midpoint of MT, A is the midpoint of SR. Prove: MS ║TR. Statements: 1.A is the midpoint of MT, A is the midpoint of SR. 2.MA ≅ TA, SA ≅ RA 3.  MAS ≅  TAR 4.∆MAS ≅ ∆TAR 5.  M ≅  T 6.MS ║ TR Reasons: 1.Given 2.Definition of a midpoint

11. Given: A is the midpoint of MT, A is the midpoint of SR. Prove: MS ║TR. Statements: 1.A is the midpoint of MT, A is the midpoint of SR. 2.MA ≅ TA, SA ≅ RA 3.  MAS ≅  TAR 4.∆MAS ≅ ∆TAR 5.  M ≅  T 6.MS ║ TR Reasons: 1.Given 2.Definition of a midpoint 3.Vertical Angles Theorem

12. Given: A is the midpoint of MT, A is the midpoint of SR. Prove: MS ║TR. Statements: 1.A is the midpoint of MT, A is the midpoint of SR. 2.MA ≅ TA, SA ≅ RA 3.  MAS ≅  TAR 4.∆MAS ≅ ∆TAR 5.  M ≅  T 6.MS ║ TR Reasons: 1.Given 2.Definition of a midpoint 3.Vertical Angles Theorem 4.SAS Congruence Postulate

13. Given: A is the midpoint of MT, A is the midpoint of SR. Prove: MS ║TR. Statements: 1.A is the midpoint of MT, A is the midpoint of SR. 2.MA ≅ TA, SA ≅ RA 3.  MAS ≅  TAR 4.∆MAS ≅ ∆TAR 5.  M ≅  T 6.MS ║ TR Reasons: 1.Given 2.Definition of a midpoint 3.Vertical Angles Theorem 4.SAS Congruence Postulate 5.Corres. parts of ≅ ∆’s are ≅

14. Given: A is the midpoint of MT, A is the midpoint of SR. Prove: MS ║TR. Statements: 1.A is the midpoint of MT, A is the midpoint of SR. 2.MA ≅ TA, SA ≅ RA 3.  MAS ≅  TAR 4.∆MAS ≅ ∆TAR 5.  M ≅  T 6.MS ║ TR Reasons: 1.Given 2.Definition of a midpoint 3.Vertical Angles Theorem 4.SAS Congruence Postulate 5.Corres. parts of ≅ ∆’s are ≅ 6.Alternate Interior Angles Converse.

15. Ex. 3: Using more than one pair of triangles. Given:  1 ≅  2,  3≅  4. Prove ∆BCE≅∆DCE Plan for proof: The only information you have about ∆BCE and ∆DCE is that  1 ≅  2 and that CE ≅CE. Notice, however, that sides BC and DC are also sides of ∆ABC and ∆ADC. If you can prove that ∆ABC≅∆ADC, you can use the fact that corresponding parts of congruent triangles are congruent to get a third piece of information about ∆BCE and ∆DCE. 2 1 4 3

16. Given:  1 ≅  2,  3 ≅  4. Prove ∆BCE ≅ ∆DCE Statements: 1.  1≅  2,  3≅  4 2.AC ≅ AC 3.∆ABC ≅ ∆ADC 4.BC ≅ DC 5.CE ≅ CE 6.∆BCE≅∆DCE Reasons: 1.Given 4 3 2 1

17. Given:  1 ≅  2,  3 ≅  4. Prove ∆BCE ≅ ∆DCE Statements: 1.  1≅  2,  3≅  4 2.AC ≅ AC 3.∆ABC ≅ ∆ADC 4.BC ≅ DC 5.CE ≅ CE 6.∆BCE≅∆DCE Reasons: 1.Given 2.Reflexive property of Congruence 4 3 2 1

18. Given:  1 ≅  2,  3 ≅  4. Prove ∆BCE ≅ ∆DCE Statements: 1.  1≅  2,  3≅  4 2.AC ≅ AC 3.∆ABC ≅ ∆ADC 4.BC ≅ DC 5.CE ≅ CE 6.∆BCE≅∆DCE Reasons: 1.Given 2.Reflexive property of Congruence 3.ASA Congruence Postulate 4 3 2 1

19. Given:  1 ≅  2,  3 ≅  4. Prove ∆BCE ≅ ∆DCE Statements: 1.  1≅  2,  3≅  4 2.AC ≅ AC 3.∆ABC ≅ ∆ADC 4.BC ≅ DC 5.CE ≅ CE 6.∆BCE≅∆DCE Reasons: 1.Given 2.Reflexive property of Congruence 3.ASA Congruence Postulate 4.Corres. parts of ≅ ∆’s are ≅ 4 3 2 1

20. Given:  1 ≅  2,  3 ≅  4. Prove ∆BCE ≅ ∆DCE Statements: 1.  1≅  2,  3≅  4 2.AC ≅ AC 3.∆ABC ≅ ∆ADC 4.BC ≅ DC 5.CE ≅ CE 6.∆BCE≅∆DCE Reasons: 1.Given 2.Reflexive property of Congruence 3.ASA Congruence Postulate 4.Corres. parts of ≅ ∆’s are ≅ 5.Reflexive Property of Congruence 4 3 2 1

21. Given:  1 ≅  2,  3 ≅  4. Prove ∆BCE ≅ ∆DCE Statements: 1.  1≅  2,  3≅  4 2.AC ≅ AC 3.∆ABC ≅ ∆ADC 4.BC ≅ DC 5.CE ≅ CE 6.∆BCE≅∆DCE Reasons: 1.Given 2.Reflexive property of Congruence 3.ASA Congruence Postulate 4.Corres. parts of ≅ ∆’s are ≅ 5.Reflexive Property of Congruence 6.SAS Congruence Postulate 4 3 2 1

22. Given: AB ≅ DE, AC ≅ DF, BC ≅ EF Prove  CAB ≅  FDE Statements: 1.AB ≅ DE 2.AC ≅ DF 3.BC ≅ EF 4.∆CAB ≅ ∆FDE 5.  CAB ≅  FDE Reasons: 1.Given 4 3 2 1

23. Given: AB ≅ DE, AC ≅ DF, BC ≅ EF Prove  CAB ≅  FDE Statements: 1.AB ≅ DE 2.AC ≅ DF 3.BC ≅ EF 4.∆CAB ≅ ∆FDE 5.  CAB ≅  FDE Reasons: 1.Given 2.Given 4 3 2 1

24. Given: AB ≅ DE, AC ≅ DF, BC ≅ EF Prove  CAB ≅  FDE Statements: 1.AB ≅ DE 2.AC ≅ DF 3.BC ≅ EF 4.∆CAB ≅ ∆FDE 5.  CAB ≅  FDE Reasons: 1.Given 2.Given 3.Given 4 3 2 1

25. Given: AB ≅ DE, AC ≅ DF, BC ≅ EF Prove  CAB ≅  FDE Statements: 1.AB ≅ DE 2.AC ≅ DF 3.BC ≅ EF 4.∆CAB ≅ ∆FDE 5.  CAB ≅  FDE Reasons: 1.Given 2.Given 3.Given 4.SSS Congruence Post 4 3 2 1

26. Given: AB ≅ DE, AC ≅ DF, BC ≅ EF Prove  CAB ≅  FDE Statements: 1.AB ≅ DE 2.AC ≅ DF 3.BC ≅ EF 4.∆CAB ≅ ∆FDE 5.  CAB ≅  FDE Reasons: 1.Given 2.Given 3.Given 4.SSS Congruence Post 5.Corres. parts of ≅ ∆’s are ≅. 4 3 2 1

27. Given: QS  RP, PT ≅ RT Prove PS ≅ RS Statements: 1.QS  RP 2.PT ≅ RT Reasons: 1.Given 2.Given 4 3 2 1