McGraw Hill Integrated II, 2012
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McGraw Hill Integrated II, 2012 View details
4. Proving Triangles Congruent-ASA, AAS
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Exercise 37 Page 371

Statements
Reasons
1.
∠ 2 ≅ ∠ 1 and ∠ 1 ≅ ∠ 3
1.
Given
2.
∠ 2 ≅ ∠ 3
2.
Transitive Property of Congruence
3.
∠ 2 and ∠ 3 are alternate interior angles
3.
From the diagram
4.
AB ∥ DE
4.
Converse of Alternate Interior Angles Theorem
Practice makes perfect
From the given diagram, we can see that BD is a transversal of AB and DE. This implies that ∠ 2 and ∠ 3 are alternate interior angles.

Since ∠ 2 ≅ ∠ 1 and ∠ 1 ≅ ∠ 3, by the Transitive Property of Congruence we get ∠ 2 ≅ ∠ 3. Next, by the Converse of Alternate Interior Angles Theorem we conclude that AB ∥ DE. We summarize this proof in the following table.

Statements
Reasons
1.
∠ 2 ≅ ∠ 1 and ∠ 1 ≅ ∠ 3
1.
Given
2.
∠ 2 ≅ ∠ 3
2.
Transitive Property of Congruence
3.
∠ 2 and ∠ 3 are alternate interior angles
3.
From the diagram
4.
AB ∥ DE
4.
Converse of Alternate Interior Angles Theorem