Answer: see below proof
Step-by-step explanation:
Imagine a diagonal line AC
If AB = DC and AB // DC then angle BAC =
Angle ACD Alternate Interior angles theorem.
Then if AB = AD and angle BAC = angle ACD and AC = AC then triangle ABC is congruent to triangle CDA.
If triangle ABC is congruent to triangle CDA then AD = BC because congruent parts of congruent triangles are congruent.
If triangle ABC is congruent to triangle CDA then angle ACB = angle CAD because congruent parts of congruent triangles are congruent.
If angle ACB = angle CAD then AD // BC because of the alternate interior angles theorem.
