∫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)