A quadrilateral in which both pairs of opposite sides are parallel is called a <u>Parallelogram</u>
Step-by-step explanation:
A quadrilateral is said to be  parallelogram if
- If its opposite sides are equal
- If the  opposite angles are equal
- If the diagonals bisect each other
- If  a pair of opposite sides is equal and parallel.
From the question given above 
 <u>ABCD is a parallelogram and P and Q are points on BD such that</u>
<u>DP=QB</u>
In ΔAPD and ΔCQB,--------------------------(i)
DP = QB (Given)
∠ADP = ∠CBQ (Alternate interior angles)
AD = BC 
(Hence it is proved that -Opposite sides of a parallelogram are equal)
so, ΔAPD ≅ ΔCQB    (As per  SAS congruence rule)
 If , ΔAPD ≅ ΔCQB.-------------------------(ii)
AP = CQ               ( by CPCT )
 In ΔAQB and ΔCPD,-----------------------(iii)
BQ = DP (Given)
∠ABQ = ∠CDP (Alternate interior angles)
AB = CD  (Opposite sides of a parallelogram)
so, ΔAQB ≅ ΔCPD                       (As per  SAS congruence rule)
(iv) AQ = CP              (According to  CPCT as ΔAQB ≅ ΔCPD.)
 From (ii)  and (iv) equation ,we can say that 
AP=CQ ,
 AQ=CP
<u> It is proved that APCQ has equal opposite sides also it has equal opposite angles. </u><u>Hence,APCQ is a Parallelogram</u>