若a、b、c都是有理数，那么2a-3b+c的相反数是（） A．3b-2a-c

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问题选择题

若a、b、c都是有理数，那么2a-3b+c的相反数是（　　）

A．3b-2a-c

B．-3b-2a+c

C．3b-2a+c

D．3b+2a-c

答案

根据相反数的定义，得2a-3b+c的相反数是-（2a-3b+c）=3b-2a-c．

故选A．

补全对话，情景问答

根据所给图示完成对话。

A: What are you doing?

B: I'm 1 my plane. Thereis something wrong 2 it. Could you help me 3 it?

A: I'm 4 . I can't help you. I'm very busy now.

B: That's all right. Well,do you 5 a knife? Mine is at home.

A: Yes, I 6 one.

B: May I 7 it?

A: Certainly. 8 . Please give it back soon.

B: I think I can give it 9 to you this afternoon

A: This afternoon? I'm not at school.

B: What 10 tomorrow?

A: That's OK.

1.______ 2.______ 3.______ 4.______ 5.______

6.______ 7.______ 8.______ 9.______ 10.______

问答题

（1）试求小球刚穿出平行金属板PQ进入磁场瞬间的速度；

（2）若要小球穿出平行金属板PQ后，经磁场偏转射入平行金属板MN中，且在不与极板相碰的前提下，最终在极板MN的左侧中点O′沿中轴线射出．则金属板Q、M间距离最大是多少？