搜题请搜索小程序“爱下问搜题”
问题 选择题
数列1，
1
1+2
，
1
1+2+3
，
1
1+2+3+4
，…，
1
1+2+…+n
的前2009项的和（　　）
A．
2009
1005
B．
4014
2008
C．
2009
2008
D．
4016
2009
答案

由于an=

1
1+2+…+n
=
1
n(n+1)
2

=

2
n(n+1)
=2（
1
n
-
1
n+1
）

则S2009=2[(1-

1
2
)+(
1
2
-
1
3
)+…+(
1
2009
-
1
2010
)]=2(1-
1
2010
)=
2×2009
2010
=
2009
1005

故答案为 A

填空题

用所给的单词的适当形式填空，使句子意思完整通顺，每条横线限填一个单词。

1. I couldn't understand why he was so               (interest) in the invitation.

2. He did the work very               (care). Everybody said he had done a good job.

3. I met an old friend of               (I) in the Summer Palace last Sunday.

4. Many new                (potato) will grow from one potato planted in the ground.

5. We hope it will be                (sun) tomorrow, for our picnic.

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多项选择题

左心室内可见（）。

A.肉柱

B. * * 肌

C.腱索

D.二尖瓣

E.主动脉口
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