问题 计算题

A、B两辆汽车在笔直的公路上同向行驶,当B车在A车前84 m处时,B车速度为4 m/s,且正以2 m/s2的加速度做匀加速运动,经过一段时间后,B车的加速度突然变为零。A车一直以20 m/s的速度做匀速运动。经过12 s后两车相遇。问B车加速行驶的时间是多少?

答案

解:设A车的速度为vA,B车加速行驶时间为t,两车在t0时相遇。则有:xA=vAt0

 ②

式中,t0=12 s,xA、xB分别为A、B两车相遇前行驶的路程,依题意有xA=xB+x ③,式中x=84 m

由①②③式得

代入题给数据vA=20 m/s,vB=4 m/s,a=2 m/s2

有t2-24t+108=0 ⑤

解得t1=6 s,t2=18 s ⑥

t2=18 s不符合题意,舍去。因此,B车加速行驶的时间为6 s

完形填空
When growing up, I hadn't the slightest curiosity about the authors of books I read; it  was the _     that was important. My mother, and sometimes my father. read _     to us every  night.   I could hardly wait       I could read and write my own books. But in the first grade,
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By the time I         fifth grade, "writing books" was still my favorite hobby. I rushed home from school each day to write down       had been forming in my head. At sixteen my  first story was       in a church magazine. In college, where I was studying to be a psychologist, was able to pay my       by writing stories. When I got my bachelor's degree, I decided to write more than anything else, so I began writing       . I have since published books for both children and       _ . I'm not happy unless I spend some time writing. Usually I write about six hours each day. I spend three months to a year on a children's book.         how well I know the       before I begin. A neovel for adults takes a year or two. When my work is going well. I wake early in the mornings, hoping it is time to get up.
小题1:
A.experienceB.storyC.processD.class
小题2:
A.aloudB.loudlyC.loudD.widely
小题3:
A.beforeB.afterC.untilD.though
小题4:
A.teachersB.parentsC.childrenD.writers
小题5:
A.LecturesB.RulesC.MeaningsD.Sentences
小题6:
A.wordsB.newsC.picturesD.promises
小题7:
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小题8:
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小题9:
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小题10:
A. set upB.told upC.made upD. put up
小题11:
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小题12:
A.armB.headC.legD.ruler
小题13:
A.arrivedB.finishedC.reachedD.managed
小题14:
A.whicheverB.howeverC.whateverD.wherever
小题15:
A.rewarded B.publishedC.recalledD.received
小题16:
A.familyB.workC.childD.tuition
小题17:
A.part-timeB.full-timeC.sometimeD.in time
小题18:
A.adultsB.menC.womenD.students
小题19:
A.figuring outB.carrying onC.depending onD.bringing out
小题20:
A.readersB.charactersC.topicsD.chapters
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