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问题 单项选择题

某溶液的pH值约为6，用酸度计测定其精密pH值时。应选择的两个标准缓冲液的 pH值是( )。

A．1.68，4.00
B．5.00，6.86
C．4.00，6.86
D．6.86，9.18
E．1.68，6.86

答案

参考答案：C

解析： pH值测定法

填空题

根据下图所示情景，补上短文中所缺单词或短语

     The (1)__________ night Jim (2)__________ very late(watched TV far into the night).When he

(3)__________ up in the morning, Jim found that it was already (4)__________ o'clock. He thought

he would be (5)__________ for school and felt very (6)__________. He (7)__________ up quickly,

took his schoolbag and (8)__________ to school without breakfast. When he (9)__________ school,

he found it was quiet and nobody else had come. He realized it was (10)__________ and he had made

a mistake.
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解答题

已知数列{an}中，a1=-1，且 （n+1）an，（n+2）an+1，n 成等差数列．

（Ⅰ）设bn=（n+1）an-n+2，求证：数列{bn}是等比数列；

（Ⅱ）求{an}的通项公式；

（Ⅲ）（仅理科做） 若an-bn≤kn对一切n∈N*恒成立，求实数k的取值范围．
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