Answer: First prove ABC is congruent to CDA, and then state AD and BC are corresponding sides of the triangles.
Explanation:
In the given picture, we have a parallelogram ABCD with diagonal AC..
AB║ CD, AD║BC
Now, By its diagonal AC it is divided in two triangles Δ ABC and Δ ADC
∠ACB=∠DAC and ∠CAB=∠ACD [alternate interior angles]
AC=AC {Reflexive property}
∴ By ASA postulate of congruence ,
Δ ABC ≅ Δ ADC
⇒ AD = BC [corresponding sides of the congruent triangles are congruent]
Hence, The sequence could be used to prove that AD = BC is
"First prove ABC is congruent to CDA, and then state AD and BC are corresponding sides of the triangles. "