So the quadrilateral abcd is a parallelogram.

So ab=cd and ad=bc.

2. because angle A= angle a, AB=AC, and angle B= angle C.

So triangle ABE is congruent with triangle ACD.

So AE=AD

Then DB=AB-AD=AC-AE=EC.

Namely DB=EC

3. Because < 3 = < 4

So ∠BOA=∠COA

Because ∠A=∠B, AO=AO, ∠BOA=∠COA.

So triangle AOB is equal to triangle AOC.

So OB=OC