证明:
∵平行四边形ABCD
∴AD=BC,AO=CO,BO=DO,AD∥BC
∴∠ADB=∠CBD
∵∠DAE=∠BCF
∴△AED≌△CFB (ASA)
∴DE=BF
∵OE=DO-DE,OF=BO-BF
∴OE=OF
∴平行四边形AECF (对角线互相平分)
证明:
∵平行四边形ABCD
∴AD=BC,AO=CO,BO=DO,AD∥BC
∴∠ADB=∠CBD
∵∠DAE=∠BCF
∴△AED≌△CFB (ASA)
∴DE=BF
∵OE=DO-DE,OF=BO-BF
∴OE=OF
∴平行四边形AECF (对角线互相平分)