∵AD=2AB且AE=BF=AB∴AF=AD∴角F=角ADF∵四边形ABCD是平行四边形∴AB//CD,AD//BC（平行四边形的对边平行）即AF//DC∴角F=角FDC,角ADC+角BCD=180°∴角F=1/2角ADC同理角E=1/2∠BCD∴∠E+∠F=1/2（∠ADC+∠BCD）=90°...