____千克
读经纬网图,回答下题。
在梯形ABCD中,AD∥BC,对角线AC⊥BD,且AC=12,BD=9,则梯形两腰中点的连线EF的长是( )
—You look so tired. What's the matter?—I was so busy that I didn't go to bed 12 o'clock last night.