证明：∵四边形ABCD是平行四边形，∴AB=CD，AB∥CD．∴∠BAE=∠DCF．又∵BE∥DF，∴∠BEF=∠EFD，∵∠BEF+∠AEB=180°，∠EFD+∠DFC=180°，∴∠AEB=∠CFD．∴△ABE≌△CDF（AAS）．∴BE=DF．∴四边形BFDE是平行四边...