∫PE∨MB，PF∨MC，

∴ Quadrilateral PEMF is a parallelogram, (3 points)

∵ parallelogram ABCD,

∴AB=CD，

∵M is the midpoint of the edge AD,

∴AM=DM= 12AD，

∫AB:AD = 1:2，

∴ab=cd=am=dm(5 points)

∴∠ABM=∠AMB, ∠DMC=∠DCM, (6 points)

∫AD∨CB，

∴∠CBM=∠AMB, DMC=∠BCM, (7 points)

∴∠cbm=∠abm= 12∠abc,∠dcm=∠bcm= 12∠dcb，

∫AD∨CB，

∴∠ ABC+∠ DCB = 180, (9 points)

∴∠MBC+∠MCB=90，

∴∠ BMC = 90, (10 point)

∴ parallelogram PEMF is rectangular. (12)