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问题 选择题

下列运算正确的是（　　）

A．3a2b-3ba2=0

B．3a+2b=5ab

C．3x2+2x3=5x5

D．3y2-2y2=1

答案

A、3a2b-3ba2=0，计算正确，故本选项正确；

B、3a和2b不是同类项，不能合并，故本选项错误；

C、3x2和2x3不是同类项，不能合并，故本选项错误；

D、3y2-2y2=y2，原式计算错误，故本选项错误．

故选A．

填空题

Motivation is "the driving force within individuals that impels them to action." And goals are the sought-after results_____(1) motivated behavior.Motivation can be either positive or negative_____(2) direction. We may feel a driving force toward some object or condition, _____(3) a driving force away from some object or condition. For example, a person may be impelled toward a restaurant to fulfill a need, hunger, and away_____(4) an airplane to fulfill a need of safety. Some psychologists refer to positive drives_____(5) needs, wants or desires,_____(6) negative drives as fears or aversions._____ (7), though negative and positive motivational forces seem to differ dramatically_____(8) terms of physical and sometimes emotional activity, they are basically similar in_____(9) they both serve to initiate and sustain human behavior.____(10) this reason, researchers often refer _____(11) both kinds of drives or motives as needs, wants and desires.Goals, _____(12), can be either positive or negative. A positive goal is one toward_____ (13) behavior is directed and it is often referred to as an approach object. A negative goal is _____(14) from which behavior is directed away and it is sometimes referred to as an avoidance object. Since both approach and avoidance goals can be considered objectives of motivated behavior, most researchers refer to_____(15) types simply as goals. Consider this example. A middle-aged woman may wish to remain_____(16) attractive as possible. Her positive goal is to appear desirable, and_____(17) she may use a perfume advertised to make her irresistible. A negative goal may be to prevent her skin_____(18) aging, and therefore she may buy and use face creams._____(19) the former case, she uses perfume to help her achieve her positive goal — attractiveness; in the____(20) case, she uses face creams to help avoid a negative goal — wrinkled skin.
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单项选择题

采用（）定位，结构简单，刚度好，但不便于更换。

A.圆柱心轴

B.圆锥心轴

C.固定定位销

D.短圆锥销
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