In △BPS,
RA || SP (PQRS is a parallelogram)
BRRS = ABAP [By BPT]
xx = ABAP
ABAP = 1
Alternatively,
Given, PQRS is a parallelogram.
⇒ PQ || RS
or PQ || BS
⇒∠APQ=∠ABR [alternate angles]
∠PAQ=∠BAR [vertically opposite angles]
⇒ΔAPQ∼ΔABR [by AA similarity]
⇒APAB=PQBR
⇒APAB=xx [as PQ = SR =x]
⇒APAB=1