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一修路工在S=100m的隧道中，突然发现一列火车出现在离右道口200m处，修路工恰在无论向右还是向左跑均能安全脱离危险的位置．问这位置离左出口的距离多少？他奔跑的速度至少是火车速度的多少倍？

答案

要脱离危险必须使人在火车到达隧道口前跑出隧道．设人所在位置离左出口为xm，知图所示，则离右出口为（100-x）m．

v人为人的速度，v车为车的速度．

从左出口人恰能脱离危险时有：

x
v人
=
300
v车

从右出口人恰能脱离危险时有：

100-x
v人
=
200
v车

由上式可得：x=60m，即人离左出口60m，

v人
v车
=
60
300
=
1
5

即人的最小速度是火车速度的

1
5
倍

答：该位置离出口60m；人的速度为火车速度的

1
5
倍．

单项选择题 A1型题

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A.细菌生命活动不可缺少的基因

B.为细菌染色体以外的遗传物质

C.具有自我复制，传给子代的特点

D.可从一个细菌转移至另一个细菌体

E.可以作为基因载体
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单项选择题

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A. databaseB. data representationC. protocolsD. data communication
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