Question

the proof of ABC = CDA is shown what is the missing reason in proof

Answer 1

the proof of ABC = CDA is shown, the missing reason in proof are:

Consider the attached figure while going through the following steps:

In Δ ABC and Δ CDA

AB ║ DC (opposite sides of a parallelogram are parallel)  

AD ║ BC (opposite sides of a parallelogram are parallel)

∠ ABD = ∠ CBD  (alternate interior angles are same)

∠ ADB = ∠ CDB   (alternate interior angles are same)

DB = DB (common side)

∴ Δ ABC ≅ Δ CDB