1.求证｛bn｝是等差数列：
2若cn≤1、4m^2+m-1对一切正整数n恒成立，求实数m的取值范围。
.已知数列{an}是首项为a1=4,公比为q的等比数列,设bn+2-3log1/4^an(n属于正实数)数列｛cn｝满足cn=an
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解决时间 2021-11-08 23:13
- 提问者网友：我没有何以琛的痴心不悔
- 2021-11-08 20:19
最佳答案
- 五星知识达人网友：一秋
- 2020-05-21 01:51
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time I did not know how to please women - has always been a female friend to please me , huh,mac cosmetics outlet, huh . Soon we became good friends then do not say no , and often went shopping together . Know her BF broke up a few years , but also know the other night to start her fierce pursuit of men .
later , she and I went to a karaoke OK party - the guy was organized in pursuit of her party. She finally agreed and the beginning of man a man .
though a bit disappointed, but I respect her , since we became friends and mutual understanding .
Soon she and the man divided.
Until now , we are still good friends ,Oakley sunglasses, she is my most important friend. In my training the next she was more confident, more wisdom.
Why bring up the past
sudden ?
because I hate the kind of relationship in order to begin gathering a certain immaturity .
Department of Why
but may be a friend known Cashmere .
the incident 4 years ago, I was left a party on the understanding of E a good friend. We are good , good to analyze the situation for a Ga people - in front of me ,Discount Oakley sunglasses, then she is like a greenhouse ga flowers, ignorant of the world . I left phone exchange , and then often send text messages .
Haojing , with wireless and a sister ah She is somewhat similar (at the time a more red lines Ga Chung Hua Dan .) When the second party , watch a lot of men on the Ga .
time I did not know how to please women - has always been a female friend to please me , huh,mac cosmetics outlet, huh . Soon we became good friends then do not say no , and often went shopping together . Know her BF broke up a few years , but also know the other night to start her fierce pursuit of men .
later , she and I went to a karaoke OK party - the guy was organized in pursuit of her party. She finally agreed and the beginning of man a man .
though a bit disappointed, but I respect her , since we became friends and mutual understanding .
Soon she and the man divided.
Until now , we are still good friends ,Oakley sunglasses, she is my most important friend. In my training the next she was more confident, more wisdom.
Why bring up the past
sudden ?
because I hate the kind of relationship in order to begin gathering a certain immaturity .
全部回答
- 1楼网友：猎心人
- 2020-03-14 14:55
解：（1）由题意，可得
an=(1/4)^n;
那么：
bn+2=3*log(1/4)an=3n;
所以：
bn=3n-2,为等差数列；
（2）由条件cn= an*bn得到：
cn= (1/4)^n*(3n-2)=3n*(1/4)^n-2*(1/4)^n
记cn的前n项和为sn；那么：
sn=3[1/4+2*(1/4)^2+……+n*(1/4)^n]-2*(1/4+(1/4)^2+……+(1/4)^n);
记pn=1/4+2*(1/4)^2+……+n*(1/4)^n; --------(1)
则有：
1/4*pn=(1/4)^2+2*(1/4)^3+……+n*(1/4)^(n+1); ------(2)
(1)-(2)得到：
3/4 pn=1/4+(1/4)^2+(1/4)^3+……+(1/4)^n-n*(1/4)^(n+1)
= 1/3*(1-(1/4)^n)- n*(1/4)^珐辅粹恍诔喝达桶惮垃(n+1)
所以sn可变形为:
sn=3[1/3*(1-(1/4)^n)- n*(1/4)^(n+1)]-2*[1/3*(1-(1/4)^n)]
=1/3*[1-(1/4)^n]-3n*(1/4)^(n+1);
【说明】在求sn的时候，用的方法是错位相减法，我记得教材里边在求等比数列和的时候用的就是这种方法；
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