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【简答题】
光滑水平面上有两辆静止的小车,它们之间用一根轻绳相互连接,第一辆车和车上人的质量总和为250千克,第二辆车的质量为500千克,现在第一辆车上的人用水平力F=25t 2 拉绳子,则3秒末第一辆车的速度大小为 ____ 米 / 秒 ,第二辆车的速度大小为 __________ 米 / 秒
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【单选题】One of the few things you _____ say about English people with certainty is that they talk a lot about the weather. [ ]
A.
can
B.
must
C.
should
D.
need
【单选题】It is implied in the passage that one of the reasons that people often talk about UFOs may be that ______.
A.
they want to tell lies
B.
they don't live in the large cities
C.
there are only few witnesses
D.
the cases are interesting although not reliable
【单选题】One of the few things you ______say about English people with certainty is that they talk a lot about the weather. [ ]
A.
need
B.
must
C.
can
D.
should
【简答题】把正方形ABCD沿其对角线AC折成直二面角D-AC-B后，连接BD，得到如图所示的几何体，已知点O、E、F分别为线段AC、AD、BC的中点。 （1）求证：AB∥平面EOF； （2）求二面角E-OF-B的大小。
【简答题】在如图所示的几何体中，四边形ABCD为正方形， 为直角三角形， ，且 . （1）证明：平面 平面 ； （2）若AB=2AE，求异面直线BE与AC所成角的余弦值.
【简答题】把正方形ABCD沿其对角线AC折成直二面角D-AC-B后，连接BD，得到如图所示的几何体，已知点D、E、F分别为线段AC、AD、BC的中点， (1)求证：AB∥平面EOF； (2)求二面角E-OF-B的大小．
【单选题】The author states that one important reason that we do not talk much about poverty is that ______.
A.
no one knows what to do about it
B.
poverty really is lower than in the past
C.
no one has been left out of the current boom
D.
the president is not concerned about the poor
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