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 | 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 |
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 |