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 9 Page 368

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 Reasons
UY∥ XW Given
∠ VYU ≅ ∠ VWX Alternate Interior Angles Theorem
V is the midpoint of YW Given
VY ≅ VW Definition of midpoint
∠ UVY ≅ ∠ XVW Vertical Angles Theorem
△ UVY ≅ △ XVW 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 Reasons
UY∥ XW Given
∠ VYU ≅ ∠ VWX Alternate Interior Angles Theorem
V is the midpoint of YW Given
VY ≅ VW Definition of midpoint
∠ UVY ≅ ∠ XVW Vertical Angles Theorem
△ UVY ≅ △ XVW Angle-Side-Angle (ASA) Congruence Postulate