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问题 解答题

已知{an}是等差数列，a3=4，a6+a9=-10，前n项和为Sn，

（1）求通项公式an

（2）当n为何值时Sn最大，并求出最大值．

答案

（1）∵{an}是等差数列，a3=4，a6+a9=-10，

∴

a1+2d=4
a 1+5d+a1+8d=-10
，

解得a1=8，d=-2，

∴an=8+（n-d）×（-2）=-2n+10．

（2）Sn=8n+

n(n-1)
2
×(-2)

=-n2+9n

=-（n-

9
2
）2+
81
4
，

∴当n=4或5时，Sn最大，最大值S4=S5=20．

句型转换

改写句子。 根据各小题A句的意思 完成B句，使两句的意思基本相同。

1. A. We had a good time in the party last week.

    B. We ______ ______ in the party last week.

2. A. Mei Xiaofang joined the League two years ago.

    B. Mei Xiaofang ______ ______ a League member for two years.

3. A. Little Tom was too young to go to school.

   B. Little Tom was not ______ ______ to go to school.

4. A. Li Lei runs faster than Wang Feng.

    B. Wang Feng doesn't run so ______ ______ Li Lei.

5. A. We found that it was easy to learn English well.

    B. We found ______ ______ to learn English well.

6. A. Mr Wang has been to Singapore? I've been there, too.

    B. Mr Wang has been to Singapore. ______ ______ I.

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单项选择题

土家族婴儿降生后父亲要用（）去岳母家报喜。

A.鸡

B.鸭

C.鱼

D.猪
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