Sn为数列{an}的前N项和,a1=2/9且an=Sn·Sn-1(n>=2)(1)求an的通项公式；(2)n为何值时,|Sn|最大?并求其最大值.

Sn为数列{an}的前N项和,a1=2/9且an=Sn·Sn-1(n>=2)(1)求an的通项公式；(2)n为何值时,|Sn|最大?并求其最大值.
Sn为数列{an}的前N项和,a1=2/9且an=Sn·Sn-1(n>=2)
(1)求an的通项公式；
(2)n为何值时,|Sn|最大?并求其最大值.

Sn为数列{an}的前N项和,a1=2/9且an=Sn·Sn-1(n>=2)(1)求an的通项公式；(2)n为何值时,|Sn|最大?并求其最大值.
由题意得：
Sn-S(n-1)=an;
1/Sn-1/S(n-1)=(S(n-1) - Sn ) / (Sn·S(n-1)) =-an/an=-1;
S1=a1=2/9;
所以数列{1/Sn}是公差为-1,首项为9/2的等差数列
所以1/Sn=1/S1+(n-1)*(-1)=11/2-n;
所以Sn=2/(11-2n);
所以an=Sn-Sn-1=4/[(2n-11)(2n-13)];
|Sn|=2/|11-2n|
要求|Sn|最大,就要求|11-2n|最小,可得当n=5或6时,|11-2n|最小是1.
即最大值是S5或S6=2

Heart-breaking farewell


Heart-breaking farewell \Parting is always to feel sad, there are \, dripping air order that \
farewell could not bear to face, but had to face. There ...

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Heart-breaking farewell


Heart-breaking farewell \Parting is always to feel sad, there are \, dripping air order that \
farewell could not bear to face, but had to face. There is unspeakable pain just parting, parting a sad heart.
howling winter wind, blowing the pain is not the hands of the cold, the sour face, but divorced in parting sinking heart.
wind shake a little of our memory sticks, so we are still clearly remember that, once the tender face, could, once the young and frivolous, had a not fluent in things, had long years unknown, we have that the simple together, knowing each other in mind, and of supporting the state of mind. Have occurred in dormitories, classrooms, the playground, those moving clips, funny scenes, and beautiful stories, stumbling, in the chaos of memory fragments of the total residue in the deep heart. Four years of acquaintance, acquaintance, and accompanied, they just between you and me on stage, acting, curtain!
chaos of snow, they float up. We do not know if this is the first few games together to see snow. The next four years, together we can see snow?
injury farewell, farewell already at hand.
say goodbye, goodbye is not too far off.
sad parting, let your eyes hang shady hack, cover the U.S. side of the sky. Desolately left only smoke, dark mist, and beyond them choke.
curtain falls, the whistle blew at the far ... ...
departure is not know where to come, He went again to the soul adrift; parting is like a bubbling spring helpless smoke slowly diffuse and sorrow, but also penetrate to the bone like the heartbreaking and the vicissitudes of life.
silence as deep as the lonely soul, bring the program full of scars of the heart, without knowing, and pity for Zhu Lei.
leaving, a silent heart full of sorrow and inexplicable sadness. So much of care, like so many friends, such as pro-anxious, now, because of the departure and can not continue.
departure is performed, parting will be ended ... ...
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Look before marriage, divorce, the former seeThat day, the clouds came two guests inside the restaurant, a man and a woman, four-year-old up and down,cheap manolo blahnik shoes uk, wearing a fairly go...

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Look before marriage, divorce, the former seeThat day, the clouds came two guests inside the restaurant, a man and a woman, four-year-old up and down,cheap manolo blahnik shoes uk, wearing a fairly good, the men also carrying a bag, seems to be a pair of couples out of travel.
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waiter told him whisper: \This odd couple.
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woman calmly said: \the river was. \bag, reached inside Meng dig up.
woman simply said, coldly: \years. I who have five dollars, enough to buy a bowl of ravioli! \You're crazy! we had no money,jimmy choo shoes, then go back a long way to how ah? \legs walked to her house. \The day when Hou. than it is now also cold! \really do not remember? \Let's first time taking a trip to do business, did not expect to be cheated up, with even travel money gone. through here, you have to give me to eat a bowl of wonton, I know, when your body on the The remaining 5 cents a ......\is here, I will never forget, then it is a small, breaking the ravioli shop. \, please let me show only the empty bowl to. \\murmured: \do not eat? \words! \mouth, straining a swallow, the whole is gargling the stomach. ravioli when he scooped a second time, tears suddenly \smiled, and began to pick up the spoon to eat. wonton entered the mouth, while tears drip into the bowl. this couple on this and tears into a bowl of wonton points finished.
down spoon, a man whispered to the rise of Woman: \think actually took out five dollars. He tuned up startled, stared at his hand could not believe the money.
woman said, smiling: \can buy a bowl of wonton, actually, you have five cents, hidden in the soles on the inside. I know you want to hide the five cents, and so when I'm hungry since then. then you are forced to eat half of ravioli, know that I certainly do not eat, put the money out and buy a bowl! \White Tibet! \
man to the woman poured a half bowl: \I just want to break up with you before then and hungry for a while. suffering a back! \: \marry eight or ten, I will not control you .....\\\\your door 20 years ago to eat ravioli that I sell, that ravioli is personally do my wife! \.
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(1)由题意得：an=Sn-Sn-1=Sn*Sn-1
所以：Sn-Sn-1/Sn*Sn-1=1
所以:1/Sn-1/Sn-1=-1
所以：{1/Sn}是以2/9为首项，以-1为公差的等差数列。
所以：1/Sn=(11-2n)/2
所以：Sn=2/(11-2n)
所以：an=Sn-Sn-1=2/(11-2n)-2/(13-2n)=4/(11-2n)(1...

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(1)由题意得：an=Sn-Sn-1=Sn*Sn-1
所以：Sn-Sn-1/Sn*Sn-1=1
所以:1/Sn-1/Sn-1=-1
所以：{1/Sn}是以2/9为首项，以-1为公差的等差数列。
所以：1/Sn=(11-2n)/2
所以：Sn=2/(11-2n)
所以：an=Sn-Sn-1=2/(11-2n)-2/(13-2n)=4/(11-2n)(13-2n)
(2)当(11-2n)最小时，Sn最大。
n=5或n=6时，Sn(max)=2

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