Side ab is parallel to side dc so the alternate interior angles, angle abd and angle bdc, are congruent. side ab is equal to side dc and db is the side common to triangles abd and bcd. therefore, the triangles abd and cdb are congruent by by cpctc, angles dbc and adb are congruent and sides ad and bc are congruent. angle dbc and angle adb form a pair of alternate interior angles. therefore, ad is congruent and parallel to bc. quadrilateral abcd is a parallelogram because its opposite sides are equal and parallel.

The  ΔABD and ΔCDB are congruent by SAS concurrency.

Step-by-step explanation:

First please take a look with diagram in attachment.

In ΔABD and ΔCDB

AB=CD                             { Given in question}

          {Alternate angle AB || CD}

BD=DB                              {Common in both triangle}

Therefor, ΔABD and ΔCDB are congruent by SAS

 by CPCT

AD=CB  by CPCT

But and are pair of alternate interior angle.

Therefore, AD parallel to CB (AD||CB)  and  AD=CB

We are given AB parallel to CD (AB||CD) and AB=CD

It means quadrilateral ABCD would be parallelogram because opposite sides are equal and parallel.

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step-by-step explanation: