Answers:
Reason 3: Definition of Parallelogram
Reason 4: Alternate Interior Angles Theorem
Reason 5: Reflexive Property of Congruence
Reason 6: ASA Congruence Property
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Explanations:
Explanation for Reason 3: A parallelogram, by definition, has opposite sides that are parallel. It's built into the name more or less. Sides AB and CD are opposite one another in the parallelogram so they are parallel segments
Explanation for Reason 4: Angle ABD is congruent to angle CDB because they are alternate interior angles. They are on the inside of the "train tracks" that are formed by AB and CD. They lay on opposite sides of the transversal BD
Explanation for Reason 5: Any segment is congruent to itself; ie, the same length
Explanation for Reason 6: Using reasons 2,5 and 4, we can use ASA (angle side angle) to prove the two triangles ABD and CDB congruent. Reason 2 is the first "A" in ASA. Reason 5 is the S in ASA. Reason 4 is the other A in ASA. The side is between the two pairs of angles. See the attache image for a visual summary of how ASA is being used.