Answers:
Reason 3: Definition of Parallelogram
Reason 4: Alternate Interior Angles Theorem
Reason 5: Reflexive Property
Reason 6: SAS Congruence Theorem
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Further Explanation:
AB || CD is true because we have a parallelogram. The opposite sides of any parallelogram are parallel.
Angle ABD = Angle CDB by the alternate interior angles theorem which basically says that if you have a pair of parallel lines, and alternate interior angles generated from a transversal, then those alternate interior angles are congruent. As you can see reason 4, depends heavily on reason 3
DB = DB is a trivial yet important statement. Its needed for SAS in line 6. This is using the reflexive property which says that any segment is congruent to itself
Triangle ABD is congruent to triangle CDB by the SAS (side angle side) triangle congruent property. This is using reasons 2, 4, and 5