If a pair of opposite sides of a quadrilateral is equal and parallel, then it is a parallelogram.
|In △ABC and △CDA|
|1. ∠BAC=∠ACD||1. Alt. ∠s, AB‖DC and AC is a transversal.
|4. △ABC≌△CDA||4. SAS axion of congruency.
Hence, ABCD is a parallelogram.
|5. c.p.c.t. Alt, ∠s are equal formed by lines AD, BC and transversal AC.