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问题 选择题
已知等差数列{an}的前n项和为Sn，且满足
S3
3
-
S2
2
=1，则数列{an}的公差是（　　）
A．
1
2
B．1C．2D．3
答案

S3=a1+a2+a3=3a1+3d，S2=a1+a2=2a1+d，

∴

S3
3
-
S2
2
=
d
2
=1

∴d=2

故选C

填空题

单词拼写。

1. You should pay attention to your behavior. After all, first ______(印象)are half the battle.

2. It's just a ______(常规的)medical examination, nothing to get worried about.

3. The ______(推荐) dosage (剂量) is one tablet every four hours.

4. If plastic and rubber are burnt, they'll give off ______(毒) gases.

5. These magnificent ancient buildings show the great ______(智慧)of the labouring people.

6. We do not like to b______ with you about the prices.

7. Please reply at your earliest c________.

8. I know him, but we don't f______ see each other.

9. C______ cars are more streamlined(流线型的) than older ones.

10. I taught in the Workers' Educational A______ in the thirties.

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选择题

下列关于反应类型的叙述中，错误的是（　　）

A．置换反应一定有单质生成

B．有氧气参加的反应都是氧化反应

C．生成盐和水的反应不一定是中和反应

D．化合反应中一定发生元素化合价的变化
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