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如图6所示，已知一个齿轮的分度圆D、齿顶圆Da、齿根圆Df，以及齿厚P/2=18mm，压力角为20°试作出此齿轮的开线齿廓线的形状。

答案

参考答案：

根据水平仪用节距法逐段测得的数据可作出6条曲线如图20所示。经计算分析，6条曲线分别如下：

平板的最大误差集中在500～1000mm段内。

多项选择题

下列属于政府职能范围的有( )。

A．企业的生产经营活动

B．打击走私

C．取缔“法轮功”

D．反垄断
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单项选择题

Until recently, most population growth has occurred in relatively dense urban environments. In (91) words, even if we were witnessing rapid population growth, that would not mean that the (92) were filling in with people. (93) , the urban areas would tend to become more crowded. Historically, we find that people have moved to (94) the jobs were. Jobs were usually in ports or locations with many natural (95) . After transportation became relatively cheap, other geographical locations became (96) of economic activity. One factor that determines where people will move to, even when transportation becomes cheap, is (97) . And individuals working in cities (98) higher incomes than those working in (99) areas. Hence, population has tended until the 1970s to (100) in our major urban centers. Recently population has (101) out of the Northeast to the South and to the West for the more pleasant climates. If people did not (102) from living in large cities, there would likely be a much more even population distribution (103) our entire land area. Probably, if this (104) the case, there would be less concern over population (105) Indeed, one need only (106) a cross-country drive or plane trip to realize how (107) populated these United States really are. This is not to say that we should or should not do something about (108) growth. The shortage of people in certain areas merely demonstrates that overpopulation in the United States is really only a problem in (109) urban environments. And this problem may be slowly correcting (110) .

A．underpopulated

B．overcrowded

C．polluted

D．inhabited
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