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问题 选择题

已知Sn是等差数列{an}的前n项和，若a3-a8+a11+a15=4，则S15-S5的值是（　　）

A．5

B．8

C．16

D．20

答案

设等差数列{an}的公差为d，则a3-a8+a11+a15

=（a1+2d）-（a1+7d）+（a1+10d）+（a1+14d）

=2a1+19d=4，

而S15-S5=15a1+

15×14
2
d-（5a1+
5×4
2
d）

=10a1+95d=5（2a1+19d）=5×4=20

故选D

阅读理解

读短文，选出正确的答案。

      Welcome to my school. There are 30 classrooms in my school. Look, this is the big playground. In this

building (大楼), you can see the gym, the library and the art room on the first floor. There are many storybooks

and picture books in the library. You can read books in it. My classroom is on the second floor. It's big and

clean. I like it.

1. There are ____________ classrooms in my school.[ ]

A. thirty

B. thirteen

2. The playground is ____________. [ ]

A. small

B. big

3. The library is on the ____________ floor.[ ]

A. first

B. second

4. My classroom is ____________.[ ]

A. clean

B. small

5. I ____________ my school.[ ]

A. don't like

B. like
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选择题

纵贯南北美洲的山系

A．喜马拉雅山系

B．阿尔卑斯山系

C．科迪勒拉山系

D．阿特拉斯山系
查看答案