平行四边形ABCD中，对角线AC、BD相交于点O，若E、F是线段AC上的两动点，分别从A、C以相同的速度1cm/s向目标C、A运动，若BD=12cm，AC=16cm，在这个运动过程中，当运动时间t=______时，四边形DEBF是矩形．
平行四边形ABCD中，AC、BD互相平分，
∵E、F分别从A、C以相同的速度1cm/s向目标C、A运动，
∴当四边形DEBF是矩形时，EF=16-2t=12，
解得t=2，
或EF=16-2（16-t）=12，
解得t=14．
综上所述，t=2秒或14秒时，四边形DEBF是矩形．
故答案为：2秒或14秒．
| 完形填空。 | |||
| I am a big peacemaker. I would much rather take the blame for just about anything than get into an 1 . However, Lose-Win ( 委曲求全) is weak. It's easy to get 2 . On and easy to gtve tn. My 3 with my mom all started one day when she said to me sarcastically(挖 苦),"Wow, you're sure 4 today."I took it so seriously that I decided to 5 , myself off from her and never talk back to her. So every time she would say something, 6 I disagreed with her I would just say,"Okay, 7 you want, Mom" But I 8 got cold quickly. One night I just 9 talking to my mom about the school homework and she said," 'Oh, that's 10 "and then went back to mop the floor. "Don't you ever 11 ?"I thought. But I didn't say anything and stormed off. She had no idea I was 13 .She would have been willing to 12 to me had I told her how important it was to me. 14 , I just blew up." Mom, this has got to 15 . You tell me everything you want me to do and I just do i' because it's 16 than quarrelling. Well, I'm sick of it." This all came as a 17 to her. After my blowup, we felt like we were starting all over in our 18 .But it's getting better all the time. We discuss things now and I always 19 my feelings with her. If you adopt Lose-Win as your basic attitude toward life, you'll be 20 your true feelings deep inside. And that's not healthy. | |||
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