4. Proving Triangles Congruent-ASA, AAS
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Use the definition of the midpoint of a segment and the Alternate Interior Angles Theorem. Notice that there is a pair of vertical angles.
Statements
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Reasons
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1. UY∥ XW
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1. Given
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2. ∠ VYU ≅ ∠ VWX
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2. Alternate Interior Angles Theorem
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3. V is the midpoint of YW
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3. Given
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4. VY ≅ VW
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4. Definition of midpoint
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5. ∠ UVY ≅ ∠ XVW
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5. Vertical Angles Theorem
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6. △ UVY ≅ △ XVW
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6. Angle-Side-Angle (ASA) Congruence Postulate
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Since V is the midpoint of YW, we get VY ≅ VW. Additionally, UY∥ XW and YW is a transversal, and so by the Alternate Interior Angles Theorem we get ∠ VYU ≅ ∠ VWX.
Notice that ∠ UVY and ∠ XVW are vertical angles which, by the Vertical Angles Theorem, we have that ∠ UVY ≅ ∠ XVW.
Therefore, by the Angle-Side-Angle (ASA) Congruence Postulate we conclude that △ UVY ≅ △ XVW. We summarize the proof in the following two-column table.
Statements
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Reasons
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1. UY∥ XW
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1. Given
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2. ∠ VYU ≅ ∠ VWX
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2. Alternate Interior Angles Theorem
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3. V is the midpoint of YW
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3. Given
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4. VY ≅ VW
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4. Definition of midpoint
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5. ∠ UVY ≅ ∠ XVW
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5. Vertical Angles Theorem
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6. △ UVY ≅ △ XVW
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6. Angle-Side-Angle (ASA) Congruence Postulate
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