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问题 单项选择题

有三个共点力F1、F2、F3作用于某一质点，其合力为零。已知F3=5N，现保持其余两力大小和方向不变，只将F3的方向沿逆时针方向绕作用点转动60°，则这三个力的合力大小变为（）

A．仍为零

B．

C．

D．

答案

参考答案：B

解析：

由共点力平衡条件知F1、F2的合力与F3平衡，F3沿逆时针方向绕作用点转动60°，则F1、F2的合力与F3的夹角为120°，由几何知识知，此时三个力的合力为5N，B正确。

填空题

(41)

Conflicts:


If you do get a place in the student dormitory, it is likely that you will have to share your living space with one other student. While having an American roommate will help you to learn more about American ways, there will probably be many times that discomfort or conflict will arise due to cultural differences.
(42)

Sex:


With regard to sex in general, American behavior is quite different from the norms found in China.
(43)

Relationship Between Teachers & Students:


On the campus, particularly where classes are small, I found a strange informality that characterized the relationship between students and their professors.
(44)

Gifts:


In my interaction with American friends, I noticed that the concept of a gift is quite different here. Many things we give to each other in China are not called "gifts" but are considered to be a reflection of ordinary duties and mutual obligations. Accustomed as we are to using the word "gift" to refer to something valuable given on special occasions, it comes as a surprise to see how often the word is used in America. In the United States, "gifts", given on many different occasions, are only services.
(45)

Social Intercourse.


When you do enter American homes, you will have an opportunity to observe different ways of greeting people. On the whole, Americans tend to be far more physical than we in their greetings.
[A] I was astonished, for example, when a friend told me that he was offering to care for his younger brother and sister so that his father could take a vacation for his birthday--this was a gift to him. For us, this would be considered duty rather than a gift. Even between friends gestures of this sort might be considered "gifts" here.
[B] While many students do call their professors, "Professor" so and so or "Dr. " so and so, some professors prefer to be called familiarly by their first names. And in the spirit of informality, many professors may invite students to their homes or can be seen chatting with students over a meal or a cup of coffee in the school cafeteria. A good number of instructors even request that students fill out class evaluation forms which assess the content and presentation of the course.
[C] My roommate was very sociable and had many boyfriends who came to visit often very late. One night, after midnight, I had to stay in the bathroom for an extra 40 minutes because I had heard a man’s voice in my room. My roommate did not realize how awkward I would feel meeting a man while I was in my nightgown. You see, American students tend to be much more casual about these matters.
[D] On many occasions, for instance, close friends or sometimes even casual acquaintances embrace or kiss each other on the cheeks in greeting or bidding farewell. It may even happen that where couples are close friends, the two husbands will kiss the other man’s wife!
[E] In the United States, for example, if a student wants to invite his teacher to a dinner party, the invitation should be sent a week or so before the party date. If the invitation is extended only three or four days before the party date, the teacher will feel he is not highly regarded.
[F] For example, many American students seem to like to listen to popular and sometimes loud music while studying in their rooms. Sometimes they will even leave the music on when they leave the room. For some reason, many will tell you, music helps them to relax and concentrate, an idea which other foreign students and I found very strange and disturbing. We’d like to study quietly without any disturbance.
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多项选择题

按投资目标划分，证券投资基金可分为( )。

A．成长型基金

B．收入型基金

C．平衡性基金

D．股票型基金
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