Let p be MN∑BC, AB intersect at M point and CD intersect at N point, then quadrilateral AMND and quadrilateral BCNM are both rectangles.

△AMP and △CNP are isosceles triangles.

∴NP=NC=MB

∠∠BPQ = 90°

∴∠ QPN+∠ BPM = 90, and ∠ BPM+∠ PBM = 90.

∴∠QPN=∠PBM .. and ∞∠QNP =∠PMB = 90°.

∴△QNP≌△PMB，

∴NQ=MP

AP = X，

∴am=mp=nq=dn=22x,bm=pn=cn= 1-22x，

∴cq=cd-dq= 1-2×22x= 1-2x

∴S△PBC= 12BC？ BM = 12× 1×( 1-22x)= 12-24x，

S△PCQ= 12CQ？ PN = 12×( 1-2x)( 1-22x)= 12-324 x+ 12 x2，

∴S quadrilateral pbcq = s△ PBC+s△ pcq =12x2-2x+1,

That is y =12x2-2x+1(0 ≤ x < 22).