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问题 单项选择题

普通轻型单吊杆的千斤索是控制（）

A.吊杆左右旋转

B.吊杆俯仰角度

C.装卸货物升降

D.固定起重柱位置

答案

参考答案：B

解答题
数列{an}的首项为a1=
5
6
，以a1，a2，a3，…，an-1，an为系数的二次方程an-1x2-anx+1=0（n≥2，且n∈N+）都有根α、β，且α、β满足3α-αβ+3β=1．
（1）求证：{an-
1
2
}是等比数列；           
（2）求{an}的通项公式；
（3）记Sn为{an}的前n项和，对一切n∈N+，不等式2Sn-n-2λ≥0恒成立，求λ的取值范围．
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改错题

Dear Mr. Black，

I’m very thank to you for inviting me to stay with you in the come summer vacation. I have been dreamed of seeing you again so that I can get help from you in my English learning. And I’m afraid I cannot go to your place in this summer. I’ve promised my grandmother, she lives in the country, to spend this summer with her. She misses me too much that I find difficult to refuse her request. I’m busy preparing for my examinations in present. I planned to visit you in winter if it’s convenient to you.

Best wishes.

Yours truly,

Li Ying
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