4.5 Using Congruent Triangles

4.5 Using Congruent Triangles Geometry Mr. Davenport Fall 2009

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

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

For example, 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. Planning 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. 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. Ex. 1: Planning & Writing a 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. Ex. 1: Paragraph 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. Ex. 2: Planning & Writing a Proof

Statements: A is the midpoint of MT, A is the midpoint of SR. MA ≅ TA, SA ≅ RA MAS ≅ TAR ∆MAS ≅ ∆TAR M ≅ T MS ║ TR Reasons: Given Given: A is the midpoint of MT, A is themidpoint of SR.Prove: MS ║TR.

Statements: A is the midpoint of MT, A is the midpoint of SR. MA ≅ TA, SA ≅ RA MAS ≅ TAR ∆MAS ≅ ∆TAR M ≅ T MS ║ TR Reasons: Given Definition of a midpoint Given: A is the midpoint of MT, A is themidpoint of SR.Prove: MS ║TR.

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

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

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

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

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. Ex. 3: Using more than one pair of triangles. 2 4 3 1 '' ie=-ia - ??? o

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

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

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

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

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

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

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

Assignment: • Pgs. 233-234 #14, 15, 17, 18, 25, 26

4.5 Using Congruent Triangles