4.2.2 等差數列的前n項和公式
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基礎知識
前n項和
等差數列\(\{a_n \}\)的首項為\(a_1\),公差為\(d\),則其前\(n\)項和為
\(S_n=\dfrac{\left(a_1+a_n\right) n}{2}\), \(S_n=n a_1+\dfrac{n(n-1)}{2} d\)
解釋
(1)證明 \(S_n=a_1+a_2+⋯+a_{n-1}+a_n\) (1)
\(S_n=a_n+a_{n-1}+⋯+a_2+a_1\) (2)
兩式相加可得\(2S_n=(a_1+a_n)+(a_2+a_{n-1})+⋯+(a_{n-1}+a_2)+(a_n+a_1)\),
有等差數列的性質:若\(m+n=s+t\), 則\(a_m+a_n=a_s+a_t\);
可得\(2S_n=(a_1+a_n )+(a_1+a_n )+⋯+(a_1+a_n )+(a_1+a_n )=n(a_1+a_n)\),
故\(S_n=\dfrac{\left(a_1+a_n\right) n}{2}\)
又\(a_n=a_1+{n-1}d\),
所以\(S_n=\dfrac{\left[a_1+a_1+(n-1) d\right] n}{2}=\dfrac{2 n a_1+n(n-1) d}{2}=n a_1+\dfrac{n(n-1)}{2} d\).
以上方法是倒序相加法.
(2)等邊數列的前\(n\)項和\(S_n=n a_1+\dfrac{n(n-1)}{2} d\),可寫成 \(S_n=\dfrac{d}{2} n^2+\left(a_1-\dfrac{d}{2}\right) n\),
當\(d≠0\)時,\(S_n\)可看成關於\(n\)的二次函式.
【例】 等差數列\(\{a_n \}\)
解析 \(S_n=\dfrac{\left(a_1+a_n\right) n}{2}=\dfrac{(1+2 n-1) n}{2}=n^2\);或\(∵a_1=1\),\(d=2\), \(\therefore S_n=n a_1+\dfrac{n(n-1)}{2} d=n^2\).
證明一個數列是等差數列的方法
① 定義法: \(a_{n+1}-a_n=d\)\((d\)是常數,\(n∈N^*)\)\(⟹a_n\)是等差數列;
② 中項法: \(2a_{n+1}=a_n+a_{n+2} (n∈N^*)⟹a_n\)是等差數列;
③ 通項公式法:\(a_n=kn+b\)\((k ,b\)是常數\() ⟹a_n\)是等差數列;
④ 前n項和公式法: \(S_n=A n^2+Bn\)\((A ,B\)是常數\() ⟹a_n\)是等差數列;
注 方法③④不可以在解答題裡直接使用.
基本性質
若數列\(\{a_n\}\)是首項為\(a_1\),公差為\(d\)的等差數列,前\(n\)項和為\(S_n\),它具有以下性質:
(1) \(S_n\) , \(S_{2 n}-S_n\), \(S_{3 n}-S_{2 n}…\)\((n∈N^*)\)成等差數列;
證明 \(S_{2 n}-S_n=a_{n+1}+a_{n+2}+\cdots+a_{2 n-1}+a_{2 n}\)
\(=\left(a_1+n d\right)+\left(a_2+n d\right)+\cdots+\left(a_{n-1}+n d\right)+\left(a_n+n d\right)=S_n+n^2 d\);
即 \(S_{2 n}-S_n-S_n=n^2 d\);
同理 \(S_{3 n}-S_{2 n}=S_{2 n}-S_n+n^2 d \Rightarrow S_{3 n}-S_{2 n}-\left(S_{2 n}-S_n\right)=n^2 d\);
\(S_{4 n}-S_{3 n}-\left(S_{3 n}-S_{2 n}\right)=n^2 d…\) 歸納得證.
【例】 \(S_n\)是一等差數列的前\(n\)項和,\(S_3\) ,\(S_6-S_3\) ,\(S_9-S_6\)成等差數列.
(2) \(S_{2 n-1}=(2 n-1) a_n\).
證明 \(S_{2 n-1}=\dfrac{(2 n-1)\left(a_1+a_{2 n-1}\right)}{2}=(2 n-1) \cdot \dfrac{a_1+a_{2 n-1}}{2}=(2 n-1) a_n\).
【例】 \(S_n\)是一等差數列的前\(n\)項和,\(S_7=7a_4\),\(S_{11}=11a_6\).
基本方法
【題型1】 等差數列前n項和的基本運算
【典題1】 記\(S_n\)為等差數列\(\{a_n\}\)的前\(n\)項和,若\(a_4+a_5=24\),\(S_6=48\),則\(S_9=\) \(\underline{\quad \quad}\).
解析 設等差數列\(\{a_n\}\)的公差為\(d\), \(\because a_4+a_5=24\),\(S_6=48\),
\(\therefore\left\{\begin{array}{c}
2 a_1+7 d=24 \\
6 a_1+\dfrac{6 \times 5}{2} d=48
\end{array}\right.\),解得\(a_1=-2\),\(d=4\),
\(\therefore S_9=9 a_1+\dfrac{9 \times 8}{2} d=-18+144=126\).
點撥 本題屬於基本量法,\(a_1\),\(d\)是等差數列基本量,遇到\(a_n\)用上\(a_n=a_1+(n-1)d\),
\(S_n\)用上 \(S_n=n a_1+\dfrac{n(n-1)}{2} d\).
【典題2】 數列\(\{a_n \}\)是等差數列,\(a_1=50\),\(d=-0.6\).
(1)該數列前多少項都是非負數?(2)求此數列的前\(n\)項和\(S_n\)的最大值.
解析 (1)由\(a_1=50\),\(d=-0.6\),
知 \(a_n=50-0.6(n-1)=-0.6n+50.6\).
令 \(\left\{\begin{array}{l}
a_m \geq 0 \\
a_{m+1}<0
\end{array}\right.\),即 \(\left\{\begin{array}{l}
-0.6 m+50.6 \geq 0 \\
-0.6(m+1)+50.6<0
\end{array}\right.\),解得 \(\dfrac{250}{3}<m \leq \dfrac{253}{3}\),
又\(m∈N^*\),則\(m=84\),
即前\(84\)項都是非負數.
(2)方法1 由(1)得\(a_{84}>0\),\(a_{85}<0\),
則\(S_n\)的最大值是 \(S_{84}=50 \times 84+\dfrac{84 \times 83}{2} \times(-0.6)=2108.4\).
方法2 \(S_n=50 n+\dfrac{n(n-1)}{2} \cdot(-0.6)=-0.3 n^2+50.3 n\)\(=-0.3\left(n-\dfrac{503}{6}\right)^2+\dfrac{503^2}{120}\),
由二次函式的性質知,當\(n=84\)時,\(S_n\)取最大值\(S_{84}=2108.4\).
點撥 求等差數列前n項和\(S_n\)的最值,顯然當等差數列遞減,\(S_n\)有最大值,可確定哪項開始為負值,便可知道最大值;當等差數列遞增,\(S_n\)有最小值,可確定哪項開始為正值,便可知道最小值;方法2是求出\(S_n\)的解析式,再利用二次函式的性質求最值.
【典題3】 等差數列\(\{a_n\}\)中,\(a_1=2020\),前\(n\)項和為\(S_n\),若 \(\dfrac{S_{12}}{12}-\dfrac{S_{10}}{10}=-2\),則\(S_{2022}=\) \(\underline{\quad \quad}\) .
解析 由等差數列前\(n\)項和為 \(S_n=n a_1+\dfrac{n(n-1)}{2} d\),
則 \(\dfrac{S_n}{n}=a_1+\dfrac{(n-1)}{2} d=-\dfrac{d}{2} n+a_1\),顯然 \(\left\{\dfrac{S_n}{n}\right\}\)為等差數列,
設 \(\left\{\dfrac{S_n}{n}\right\}\)公差為\(d_1\),
\(\because a_1=2020\), \(\therefore \dfrac{S_1}{1}=\dfrac{a_1}{1}=2020\),
\(\because \dfrac{S_{12}}{12}-\dfrac{S_{10}}{10}=2 d_1=-2\),\(\therefore d_1=-1\),
\(\therefore \dfrac{s_{2022}}{2022}=2020+2021 \times(-1)=-1\),解得\(S_{2022}=-2022\).
點撥 等差數列中,前\(n\)項和為\(S_n\),則\(\left\{\dfrac{S_n}{n}\right\}\)為等差數列.
【鞏固練習】
1.已知\(\{a_n\}\)為等差數列,若\(a_3=1\),\(S_4=0\),則\(a_6\)的值為( )
A.\(6\) \(\qquad \qquad \qquad \qquad\) B.\(7\) \(\qquad \qquad \qquad \qquad\) C.\(8\) \(\qquad \qquad \qquad \qquad\) D.\(9\)
2.(多選)設數列\(\{a_n\}\)是等差數列,\(S_n\)是其前\(n\)項和,\(a_1>0\)且\(S_6=S_9\),則( )
A.\(d>0\) \(\qquad \qquad \qquad \qquad\) B.\(a_8=0\) \(\qquad \qquad \qquad \qquad\) C.\(S_7\)或\(S_8\)為\(S_n\)的最大值 \(\qquad \qquad \qquad \qquad\) D.\(S_5>S_6\)
3.已知在等差數列\(\{a_n\}\)中,\(a_3=12\),\(S_{12} S_{13}<0\),則\(S_n\)最大時\(n=\) \(\underline{\quad \quad}\).
參考答案
-
答案 \(B\)
解析 設等差數列\(\{a_n\}\)的公差為\(d\),
由 \(\left\{\begin{array}{l} a_3=1 \\ S_4=0 \end{array}\right.\),得 \(\left\{\begin{array}{l} a_1+2 d=1 \\ 4 a_1+6 d=0 \end{array}\right.\),解得 \(\left\{\begin{array}{l} a_1=-3 \\ d=2 \end{array}\right.\),
所以\(a_6=-3+2×5=7\).
故選:\(B\). -
答案 \(BC\)
解析 \(a_1>0\)且\(S_6=S_9\),
\(\therefore 6 a_1+\dfrac{6 \times 5}{2} d=9 a_1+\dfrac{9 \times 8}{2} d\),化為:\(a_1+7d=0\),
可得\(a_8=0\),\(d<0\).
\(S_7\)或\(S_8\)為\(S_n\)的最大值,\(S_5<S_6\).
故選:\(BC\). -
答案 \(6\)
解析 設等差數列\(\{a_n\}\)的公差為\(d\),\(\because a_3=12\),\(S_{12} S_{13}<0\),
\(\therefore a_1+2d=12\), \(\left(12 a_1+\dfrac{12 \times 11}{2} d\right)\left(13 a_1+\dfrac{13 \times 12}{2} d\right)<0\),
化為: \((d+3)\left(d+\dfrac{24}{7}\right)<0\),
解得\(-\dfrac{24}{7}<d<-3\),可得:\(\dfrac{7}{2}<-\dfrac{12}{d}<4\).
因此等差數列\(\{a_n\}\)單調遞減,
\(\therefore S_{12}>0\),\(S_{13}<0\).
\(a_n=a_1+(n-1)d=12-2d+(n-1)d=12+(n-3)d≥0\),
可得 \(n \leq 3-\dfrac{12}{d}\),
\(\because \dfrac{13}{2} \leq 3-\dfrac{12}{d} \leq 7\),\(\therefore n≤6\).
則\(S_n\)最大時\(n=6\).
【題型2】等差數列前n項和的性質
【典題1】 已知兩個等差數列\(\{a_n \}\),\(\{b_n \}\),它們的前\(n\)項和分別記為\(S_n\),\(T_n\),若 \(\dfrac{S_n}{T_n}=\dfrac{n+3}{n+1}\),求 \(\dfrac{a_{10}}{b_{10}}\) .
解析 在等差數列\(\{a_n \}\),\(\{b_n \}\)中 \(\because \dfrac{S_{19}}{T_{19}}=\dfrac{19 a_{10}}{19 b_{10}}=\dfrac{a_{10}}{b_{10}}\),
\(\therefore \dfrac{a_{10}}{b_{10}}=\dfrac{S_{19}}{T_{19}}=\dfrac{19+3}{19+1}=\dfrac{11}{10}\).
點撥 性質 \(S_{2 n-1}=(2 n-1) a_n\)的運用.
【典題2】 一個等差數列的前\(n\)項和為\(S_n\),\(S_{10}=100\), \(S_{100}=10\),求\(S_{110}\).
解析 方法1 設等差數列\(\{a_n \}\)的公差為\(d\),前\(n\)項和為\(S_n\),
則 \(S_n=n a_1+\dfrac{n(n-1)}{2} d\).
由已知,得 \(\left\{\begin{array}{l}
10 a_1+\dfrac{10 \times 9}{2} d=100 \\
100 a_1+\dfrac{100 \times 99}{2} d=10
\end{array}\right.\),解得 \(d=-\dfrac{11}{50}\), \(a_1=\dfrac{1099}{100}\),
則 \(S_{110}=110 a_1+\dfrac{110 \times 109}{2} d=110 \times \dfrac{1099}{100}+\dfrac{110 \times 109}{2} \times\left(-\dfrac{11}{50}\right)\)\(=110 \times\left(\dfrac{1099-109 \times 11}{100}\right)=-110\).
故此數列的前\(110\)項之和為\(-110\).
方法2 設此等差數列的前\(n\)項和為\(S_n=an^2+bn\).
\(\because S_{10}=100\), \(S_{100}=10\),
\(\therefore\left\{\begin{array}{l}
10^2 a+10 b=100 \\
100^2 a+100 b=10
\end{array}\right.\),解得 \(\left\{\begin{array}{l}
a=-\dfrac{11}{100} \\
b=\dfrac{111}{10}
\end{array}\right.\),
\(\therefore S_n=-\dfrac{11}{100} n^2+\dfrac{111}{10} n\).
\(\therefore S_{110}=-\dfrac{11}{100} \times 110^2+\dfrac{111}{10} \times 110=-110\).
方法3 數列 \(S_{10}, S_{20}-S_{10}, S_{30}-S_{20}, \ldots, S_{100}-S_{90}, S_{110}-S_{100}\)成等差數列.
設其公差為\(D\),則前\(10\)項的和為 \(10 S_{10}+\dfrac{10 \times 9}{2} \cdot D=S_{100}=10\),解得\(D=-22\),
\(\therefore S_{110}-S_{100}=S_{10}+(11-1) D=100+10 \times(-22)=-120\).
\(\therefore S_{110}=-120+S_{100}=-110\).
方法4 \(\because S_{100}-S_{10}=a_{11}+a_{12}+\cdots+a_{100}=\dfrac{90\left(a_{11}+a_{100}\right)}{2}=\dfrac{90\left(a_1+a_{110}\right)}{2}\),
又 \(S_{100}-S_{10}=10-100=-90\), \(\therefore a_1+a_{110}=-2\).
\(\therefore S_{110}=\dfrac{110\left(a_1+a_{110}\right)}{2}=-110\).
點撥 注意比較各種方法的優劣,掌握等差數列的基本性質.
【鞏固練習】
1.等差數列\(\{a_n\}\)的前\(n\)項和為\(S_n\),若\(a_2+a_8+a_{11}=60\),則\(S_{13}\)的值是( )
A.\(130\) \(\qquad \qquad \qquad \qquad\) B.\(260\) \(\qquad \qquad \qquad \qquad\) C.\(390\) \(\qquad \qquad \qquad \qquad\) D.\(520\)
2.已知數列\(\{a_n\}\)為等差數列,\(S_n\)為其前\(n項\)和,\(2+a_5=a_6+a_3\),則\(S_7=\)( )
A.\(2\) \(\qquad \qquad \qquad \qquad\) B.\(7\) \(\qquad \qquad \qquad \qquad\) C.\(14\) \(\qquad \qquad \qquad \qquad\) D.\(28\)
3.已知等差數列\(\{a_n\}\)的前\(n\)項和為\(S_n\),\(S_4=3\),\(S_{n-4}=12(n≥5,n∈N^*)\),\(S_n=17\),則\(n\)的值為( )
A.\(8\) \(\qquad \qquad \qquad \qquad\) B.\(11\) \(\qquad \qquad \qquad \qquad\) C.\(13\) \(\qquad \qquad \qquad \qquad\) D.\(17\)
4.等差數列\(\{a_n\}\),\(\{b_n\}\)的前\(n\)項和分別為\(S_n\),\(T_n\) ,若\(\dfrac{S_n}{T_n}=\dfrac{2 n}{3 n+1}\),則 \(\dfrac{a_n}{b_n}=\)( )
A. \(\dfrac{2}{3}\) \(\qquad \qquad \qquad \qquad\) B. \(\dfrac{2 n-1}{3 n-1}\) \(\qquad \qquad \qquad \qquad\) C. \(\dfrac{2 n+1}{3 n+1}\) \(\qquad \qquad \qquad \qquad\) D. \(\dfrac{2 n-1}{3 n+4}\)
5.設\(S_n\)是等差數列\(\{a_n\}\)的前n項和,若 \(\dfrac{S_3}{S_6}=\dfrac{1}{3}\),則\(\dfrac{S_6}{S_{12}}=\)( )
A. \(\dfrac{3}{10}\) \(\qquad \qquad \qquad \qquad\) B. \(\dfrac{1}{3}\) \(\qquad \qquad \qquad \qquad\) C. \(\dfrac{1}{8}\) \(\qquad \qquad \qquad \qquad\) D. \(\dfrac{1}{9}\)
參考答案
-
答案 \(B\)
解析 設等差數列\(\{a_n\}\)的公差為\(d\),
\(\because a_2+a_8+a_{11}=60\),
\(\therefore a_1+d+a_1+7d+a_1+10d=3(a_1+6d)=3a_7=60\),解得\(a_7=20\),
\(\therefore S_{13}=\dfrac{13\left(a_1+a_{13}\right)}{2}=13 a_7=13 \times 20=260\).
故選:\(B\). -
答案 \(C\)
解析 \(\because 2+a_5=a_6+a_3\),\(\therefore a_4=a_6+a_3-a_5=2\).
則 \(S_7=\dfrac{7\left(a_1+a_7\right)}{2}=7 a_4=14\).故選:\(C\). -
答案 \(D\)
解析 由題意可得,\(S_4=a_1+a_2+a_3+a_4=3\) ①,
\(\because S_{n-4}=12\),\(S_n=17\),
\(\therefore a_{n-3}+a_{n-2}+a_{n-1}+a_n=17-12=5\)②,
①+②可得, \(\left(a_1+a_n\right)+\left(a_2+a_{n-1}\right)+\left(a_3+a_{n-2}\right)+\left(a_4+a_{n-3}\right)=8\),
\(\therefore a_1+a_n=2\),
\(\because S_n=17\), \(\therefore S_n=\dfrac{n\left(a_1+a_n\right)}{2}=17\),解得\(n=17\).
故選:\(D\). -
答案 \(B\)
解析 \(\dfrac{a_n}{b_n}=\dfrac{2 a_n}{2 b_n}=\dfrac{a_1+a_{2 n-1}}{b_1+b_{2 n-1}}=\dfrac{\dfrac{1}{2}(2 n-1)\left(a_1+a_{2 n-1}\right)}{\dfrac{1}{2}(2 n-1)\left(b_1+b_{2 n-1}\right)}\)\(=\dfrac{S_{2 n-1}}{T_{2 n-1}}=\dfrac{2(2 n-1)}{3(2 n-1)+1}=\dfrac{2 n-1}{3 n-1}\),故選\(B\). -
答案 \(A\)
解析 方法一 \(\because \dfrac{S_3}{S_6}=\dfrac{1}{3}\), \(\therefore \dfrac{3 a_1+3 d}{6 a_1+15 d}=\dfrac{1}{3}\),化簡得\(a_1=2d\),
\(\therefore \dfrac{S_6}{S_{12}}=\dfrac{6 a_1+15 d}{12 a_1+66 d}=\dfrac{27 d}{90 d}=\dfrac{3}{10}\).故選\(A\).
方法二 \(\because \dfrac{S_3}{S_6}=\dfrac{1}{3}\),令\(S_3=1\), \(S_6=3\),
\(\because S_3, S_6-S_3, S_9-S_6, S_{12}-S_9\)成等差數列,
\(\therefore 1,2,S_9-3,S_{12}-S_9\)成等差數列,
顯然這等差數列公差為\(1\),所以\(S_9-3=3\),\(S_{12}-S_9=4\),
解得\(S_{12}=10\), \(\therefore \dfrac{S_6}{S_{12}}=\dfrac{3}{10}\),故選\(A\).
【題型2】 等差數列前n項和的綜合
【典題1】 已知等差數列\(\{a_n\}\)滿足:\(a_1=2\),\(a_5=18\).
(1)求數列\(\{a_n\}\)的通項公式;
(2)記\(S_n\)為數列\(\{a_n\}\)的前n項和,求正整數\(n\)的範圍,使得\(S_n>60n+800\).
解析 (1)設等差數列\(\{a_n\}\)的公差為\(d\),
則\(4d=a_5-a_1=18-2=16\),解得\(d=4\),
故\(a_n=a_1+{n-1}d=2+4{n-1}=4n-2\).
(2) \(S_n=\dfrac{n[2+(4 n-2)]}{2}=2 n^2\),
令\(2n^2>60n+800\),即\(n^2-30n-400>0\),解得\(n>40\)或\(n<-10\)(捨去),
故存在正整數\(n\),使得\(S_n>60n+800\)成立,\(n\)的最小值為\(41\).
【典題2】 某長江抗洪指揮部接到預報,\(24\)小時後有一洪峰到達.為確保安全,指揮部決定在洪峰來臨前築一道堤壩作為第二道防線.經計算,除現有的部隊指戰員和當地幹部群眾連續奮戰外,還需用\(20\)臺同型號的翻斗車,平均每輛車要工作\(24\)小時才能完成任務.但目前只有一輛車投入施工,其餘的需從附近高速公路上抽調,每隔\(20\)分能有一輛車到達,且指揮部最多還可調集\(24\)輛車,那麼在\(24\)時內能否構築成第二道防線?
解析 設第\(n\)輛車工作的時間是\(a_n\)小時,
則有\(a_n-a_{n+1}=\dfrac{20}{60}=\dfrac{1}{3}\) (小時),
所以數列\(\{a_n\}\)是等差數列,公差 \(d=-\dfrac{1}{3}\),\(a_1=24\).
如果把所有的\(25\)輛車全部抽調到位,所用的時間是 \(\dfrac{20}{60} \times 24=8\) (小時)\(<24\)小時,
則這\(25\)輛車可以完成的工作量為
\(S_{25}=a_1+a_2+\cdots+a_{25}=25 a_1+\dfrac{25 \times(25-1)}{2} d\)
\(=25 \times 24+\dfrac{25 \times 24}{2} \times\left(-\dfrac{1}{3}\right)=500\)(小時).
總共需要完成的工作量為\(24×20=480\)(小時).
由於\(500>480\),
所以,在\(24\)小時內能構築成第二道防線.
【鞏固練習】
1.某景區三絕之一的鐵旗杆鑄於道光元年,兩根分別立於人口兩側,每根重約\(12000\)斤,旗杆分五節,每節分鑄八卦龍等圖案,每根杆,上還懸掛\(24\)只玲瓏的鐵風鈴.已知每節長度約成等差數列,第一節長約\(12\)尺,總長約\(48\)尺,則第五節長約為幾尺( )
A.\(7\) \(\qquad \qquad \qquad \qquad\) B.\(7.2\) \(\qquad \qquad \qquad \qquad\) C.\(7.6\) \(\qquad \qquad \qquad \qquad\) D.\(8\)
2.公差不為零的等差數列\(\{a_n\}\)滿足\(a_3=a_5 a_8\),\(a_6=1\).
(1)求\(\{a_n\}\)的通項公式;
(2)記\(\{a_n\}\)的前n項和為\(S_n\),求使\(S_n<a_n\)成立的最大正整數\(n\).
3.在正項等差數列\(\{a_n \}\)中,其前\(n\)項和為\(S_n\),\(a_2+a_3=12\),\(a_2⋅a_3=S_5\).
(1)求\(a_n\);
(2)證明: \(\dfrac{1}{3} \leq \dfrac{1}{S_1}+\dfrac{1}{S_2}+\cdots+\dfrac{1}{S_n}<\dfrac{3}{4}\).
參考答案
-
答案 \(B\)
解析 設每旗杆節長度成等差數列\(\{a_n\}\),其公差為\(d\),
由題意 \(\left\{\begin{array}{l} a_1=12 \\ S_5=5 a_1+10 d=48 \end{array}\right.\),則\(60+10d=48\),即\(d=-1.2\),
所以\(a_5=a_1+4d=12+4×(-1.2)=7.2\).
所以第五節長為\(7.2\)尺.
故選:\(B\). -
答案 (1) \(a_n=2n-11(n∈N^* )\); (2)\(10\).
解析 (1)設等差數列\(\{a_n\}\)的公差為\(d(d≠0)\),
由\(a_3=a_5 a_8\),\(a_6=1\),得\(1-3d=(1-d)(1+2d)\),
即\(2d^2-4d=0\),解得\(d=2\),或\(d=0\)(捨去),
則\(a_1=a_6-5d=1-10=-9\),
所以\(a_n=-9+2(n-1)=2n-11(n∈N^* )\);
(2)由(1)可知 \(S_n=\dfrac{n}{2}\left(a_1+a_n\right)=\dfrac{n}{2}(-9+2 n-11)=n^2-10 n\),
令\(S_n<a_n\),得\(n^2-10n<2n-11\),
即\(n^2-12n+11<0\),解得\(1<n<11\),
又\(n∈N^*\),
故使\(S_n<a_n\)成立的最大正整數\(n\)為\(10\). -
答案 (1) \(a_n=2n+1\);(2)略.
解析 (1) \(\because \left\{\begin{array}{l} a_2+a_3=12 \\ a_2 \cdot a_3=S_5=5 a_3 \end{array}\right.\),
即 \(\left\{\begin{array}{l} a_2=a_1+d=5 \\ a_3=a_1+2 d=7 \end{array}\right.\),解得\(a_1=3\),\(d=2\),
\(\therefore a_n=2n+1\).
證明:(2)\(S_n=n(n+2)\), \(\dfrac{1}{S_n}=\dfrac{1}{n(n+2)}=\dfrac{1}{2}\left(\dfrac{1}{n}-\dfrac{1}{n+2}\right)\),
\(\therefore \dfrac{1}{S_1}+\dfrac{1}{S_2}+\cdots+\dfrac{1}{S_n}=\dfrac{1}{2}\left(1+\dfrac{1}{2}+\cdots \ldots-\dfrac{1}{n+1}-\dfrac{1}{n+2}\right)<\dfrac{3}{4}\),
當\(n=1\)時,取最大值\(\dfrac{1}{3}\) ,
綜上: \(\dfrac{1}{3} \leq \dfrac{1}{S_1}+\dfrac{1}{S_2}+\cdots+\dfrac{1}{S_n}<\dfrac{3}{4}\).
分層練習
【A組---基礎題】
1.等差數列\(\{a_n \}\)的公差\(d=2\),\(a_1=1\),則( )
A.\(a_n=2n,S_n=n^2\) \(\qquad \qquad \qquad \qquad\) B.\(a_n=n,S_n=n^2+n\) \(\qquad \qquad \qquad \qquad\)
C.\(a_n=2n-1,S_n=n^2\) \(\qquad \qquad \qquad \qquad\) D.\(a_n=2n-1,S_n=n^2-n\)
2.在等差數列\(\{a_n\}\)中\(a_{10}=2a_8-2\),則數列\(\{a_n\}\)的前\(11\)項的和\(S_{11}=\)( )
A.\(8\) \(\qquad \qquad \qquad \qquad\) B.\(16\) \(\qquad \qquad \qquad \qquad\) C.\(22\) \(\qquad \qquad \qquad \qquad\) D.\(44\)
3.等差數列\(\{a_n\}\)的通項公式\(a_n=2n+1\)其前\(n\)項和為\(S_n\),則數列\(\left\{\dfrac{S_n}{n}\right\}\)前\(10\)項的和為( )
A.\(120\) \(\qquad \qquad \qquad \qquad\) B.\(70\) \(\qquad \qquad \qquad \qquad\) C.\(75\) \(\qquad \qquad \qquad \qquad\) D.\(100\)
4.已知等差數列\(\{a_n\}\)的前\(n\)項和為\(S_n\),\(S_4=40\),\(S_n=210\), \(S_{n-4}=130\),則\(n=\)( )
A.\(12\) \(\qquad \qquad \qquad \qquad\) B.\(14\) \(\qquad \qquad \qquad \qquad\) C.\(16\) \(\qquad \qquad \qquad \qquad\) D.\(18\)
5.中國古詩詞中,有一道“八子分綿”的數學名題:“九百九十六斤綿,贈分八子作盤纏,次第每人多十七,要將第八數來言”.題意是:把\(996\)斤綿分給\(8\)個兒子作盤纏,按照年齡從大到小的順序依次分綿,年齡小的比年齡大的多\(17\)斤綿,那麼第\(8\)個兒子分到的綿是( )
A.\(201\)斤 \(\qquad \qquad \qquad \qquad\) B.\(191\)斤 \(\qquad \qquad \qquad \qquad\) C.\(184\)斤\(\qquad \qquad \qquad \qquad\) D.\(174\)斤
6.(多選)等差數列\(\{a_n\}\)的前n項和為\(S_n\),\(a_1+5a_3=S_8\),則下列結論一定正確的是( )
A.\(a_{10}=0\) \(\qquad \qquad \qquad \qquad\) B.當\(n=9\)或\(10\)時,\(S_n\)取最大值 \(\qquad \qquad \qquad \qquad\)
C.\(|a_9 |<|a_{11}|\) \(\qquad \qquad \qquad \qquad\) D.\(S_6=S_{13}\)
7.已知等差數列\(\{a_n\}\)的前n項和為\(S_n\),若 \(S_{10}=110\), \(S_{110}=10\),則 \(S_{120}=\) \(\underline{\quad \quad}\).
8.設等差數列\(\{a_n\}\)的前n項和為\(S_n\),若\(a_6=6\),\(S_{15}=15\),則公差\(d=\) \(\underline{\quad \quad}\).
9.已知等差數列\(\{a_n\}\)的公差\(d>0\),\(a_2=-11\),\(a_5^2-a_{10}^2=0\),則\(S_{15}=\) \(\underline{\quad \quad}\).
10.記\(S_n\)為等差數列\(\{a_n\}\)的前\(n\)項和,已知\(a_1=-3\),\(S_4=0\).
(1)求\(\{a_n \}\)的通項公式\(a_n\)和\(S_n\);
(2)求\(a_2+a_4+⋯+a_8+a_{10}+a_{12}\)的值.
11.已知\(S_n\)為等差數列\(\{a_n \}\)的前\(n\)項和,已知\(S_2=2\),\(S_3=-6\).
(1)求數列\(\{a_n \}\)的通項公式和前\(n\)項和\(S_n\);
(2)是否存在\(n\),使\(S_n\),\(S_{n+2}+2n\),\(S_{n+3}\)成等差數列,若存在,求出\(n\),若不存在,說明理由.
參考答案
-
答案 \(C\)
-
答案 \(C\)
解析 在等差數列\(\{a_n\}\)中,由\(a_10=2a_8-2\),得\(a_1+9d=2a_1+14d-2\),
可得\(a_1+5d=a_6=2\),\(\therefore S_{11}=11a_6=22\).
故選:\(C\). -
答案 \(C\)
解析 \(\because a_n=2n+1\), \(\therefore S_n=\dfrac{n\left(a_1+a_n\right)}{2}=n^2+2 n\) ,
\(\therefore \dfrac{S_n}{n}=n+2\),
所以數列\(\left\{\dfrac{S_n}{n}\right\}\)也是等差數列,且通項公式為\(n+2\),
則首項為\(3\),第\(10\)項為\(12\),
所以前\(10\)項的和 \(\dfrac{10(3+12)}{2}=75\). -
答案 \(B\)
解析 因為\(S_4=40\),所以\(a_1+a_2+a_3+a_4=40\),
因為 \(S_n-S_{n-4}=80\),所以 \(a_n+a_{n-1}+a_{n-2}+a_{n-3}=80\),
所以根據等差數列的性質可得:\(4(a_1+a_n)=120\),即\(a_1+a_n=30\).
由等差數列的前n項和的公式可得: \(S_n=\dfrac{n\left(a_1+a_n\right)}{2}\),並且\(S_n=210\),
所以解得\(n=14\).
故選:\(B\). -
答案 \(C\)
解析 用\(a_1,a_2,...,a_8\)是表示\(8\)個兒子按照年齡從大到小得到的綿數,
由題意得數列\(a_1,a_2,...,a_8\)是公差為\(17\)的等差數列,且這\(8\)項的和為\(996\),
\(\therefore 8 a_1+\dfrac{8 \times 7}{2} \times 17=996\),解得\(a_1=65\),
\(\therefore a_8=65+7×17=184\).
\(\therefore\)第\(8\)個兒子分到的綿是\(184\)斤.
故選:\(C\). -
答案 \(AD\)
解析 \(\because\) 等差數列\(\{a_n\}\)的前\(n\)項和為\(S_n\),\(a_1+5a_3=S_8\),
\(\therefore a_1+5\left(a_1+2 d\right)=8 a_1+\dfrac{8 \times 7}{2} d\),求得\(a_1=-9d\).
故\(a_10=a_1+9d=0\),故\(A\)正確;
該數列的前\(n\)項和 \(S_n=n a_1+\dfrac{n(n-1)}{2} d=\dfrac{n^2}{2}-d-\dfrac{19}{2} d n\),
它的最值,還跟\(d\)的值有關,
不能推出當\(n=9\)或\(10\)時,\(S_n\)取最大值,故\(B\)錯誤.
\(\because |a_9 |=|a_1+8d|=|-d|=|d|\),\(|a_{11} |=|a_1+10d|=|d|\),
故有\(|a_9 |=|a_{11}|\),故\(C\)錯誤;
由於 \(S_6=6 a_1+\dfrac{6 \times 5}{2} d=-39 d\), \(S_{13}=13 a_1+\dfrac{13 \times 12}{2} d=-39 d\),
故\(S_6=S_{13}\),故\(D\)正確,
故選:\(AD\). -
答案 \(-120\)
解析 令 \(b_n=\dfrac{s_n}{n} \text {, }\),
\(\because \{a_n\}\)是等差數列,\(\therefore \{b_n\}\)也是等差數列,設其公差為\(d\),
則 \(b_{10}=\dfrac{S_{10}}{10}=11\), \(b_{110}=\dfrac{S_{110}}{110}=\dfrac{10}{110}=\dfrac{1}{11}\),
\(\therefore 100 d=b_{110}-b_{10}=\dfrac{1}{11}-11=-\dfrac{120}{11}\),解得 \(d=-\dfrac{6}{55}\),
\(\therefore b_{120}=b_{110}+10 d=\dfrac{1}{11}-\dfrac{6}{55} \times 10=-1\),
\(\therefore \dfrac{S_{120}}{120}=-1\),即 \(S_{120}=-120\). -
答案 \(-\dfrac{5}{2}\)
解析 \(\because a_6=6\),\(S_{15}=15\),
\(\therefore a_1+5d=6\), \(15 a_1+\dfrac{15 \times 14}{2} d=15\), \(\therefore d=-\dfrac{5}{2}\). -
答案 \(15\)
解析 等差數列\(\{a_n\}\)的公差\(d>0\),\(a_2=-11\),\(a_5^2-a_{10}^2=0\),
\(\therefore a_5=-a_{10}<0\).
\(\therefore a_1+d=-11\),\(a_1+4d=-(a_1+9d)\),解得:\(a_1=-13\),\(d=2\).
則 \(S_{15}=-13 \times 15+\dfrac{15 \times 14}{2} \times 2=15\).
故答案為:\(15\). -
答案 (1) \(a_n=2n-5\),\(S_n=n^2-4n\); (2)\(54\).
解析 設等差數列\(\{a_n \}\)的公差為\(d\),
由\(a_1=-3\),\(S_4=0\),得\(4a_1+6d=-12+6d=0\),即\(d=2\).
(1)\(a_n=-3+2(n-1)=2n-5\), \(S_n=-3 n+\dfrac{n(n-1)}{2} \times 2=n^2-4 n\);
(2) \(a_2+a_4+\cdots+a_8+a_{10}+a_{12}=-6+\dfrac{6 \times 5}{2} \times 4=54\). -
答案 (1)\(a_n=10-6n\),\(S_n=7n-3n^2\);
(2)存在\(n=5\),使\(S_n\),\(S_{n+2}+2n\),\(S_{n+3}\)成等差數列.
解析 (1)設等差數列\(\{a_n \}\)的公差為\(d\),\(\because S_2=2\),\(S_3=-6\).
\(\therefore 2a_1+d=2\),\(3a_1+3d=-6\),解得\(a_1=4\),\(d=-6\).
\(\therefore a_n=4-6(n-1)=10-6n\).
\(S_n=\dfrac{n(4+10-6 n)}{2}=7 n-3 n^2\).
(2)假設存在\(n\),使\(S_n\),\(S_{n+2}+2n\),\(S_{n+3}\)成等差數列,
則 \(12\left(S_{n+2}+2 n\right)=S_n+S_{n+3}\),
\(\therefore 2[7{n+2}-3{n+2}^2+2n]=7n-3n^2+7(n+3)-3(n+3)^2\),
解得\(n=5\).
因此存在\(n=5\),使\(S_n\),\(S_{n+2}+2n\),\(S_{n+3}\)成等差數列.
【B組---提高題】
- 已知\(S_n\)是等差數列\(\{a_n\}\)的前\(n\)項和,且\(S_6>S_7>S_5\),給出下列五個命題:
①\(d<0\); \(\qquad \qquad\) ②\(S_{11}>0\);\(\qquad \qquad\) ③\(S_{12}<0\);\(\qquad \qquad\)
④數列\(\{S_n\}\)中的最大項為\(S_{11}\); \(\qquad \qquad\) ⑤\(|a_6 |>|a_7 |\).
其中正確命題的個數是 ( )
A.\(3\) \(\qquad \qquad \qquad \qquad\) B.\(4\) \(\qquad \qquad \qquad \qquad\) C.\(5\) \(\qquad \qquad \qquad \qquad\) D.\(1\)
2.在數列\(\{a_n\}\)中,\(a_{n+2}-a_n=2(n∈N^* )\),\(a_1=-23\),\(a_2=-19\),\(S_n\)為\(\{a_n\}\)的前\(n\)項和,則\(S_n\)的最小值為\(\underline{\quad \quad}\).
3.遞減的等差數列\(\{a_n\}\)的前\(n\)項和為\(S_n\),若\(a_3 a_5=63\),\(a_2+a_6=16\)
(1)求\(\{a_n\}\)的等差通項;
(2)當\(n\)為多少時,\(S_n\)取最大值,並求出其最大值;
(3)求\(|a_1 |+|a_2 |+|a_3 |+⋯+|a_n |\).
參考答案
-
答案 \(A\)
解析 \(\because S_6>S_7>S_5\),
\(\therefore a_6=S_6-S_5>0\),\(a_7=S_7-S_6<0\),\(a_6+a_7=S_7-S_5>0\),
①\(d=a_7-a_6<0\) , 所以①正確;
② \(S_{11}=\dfrac{11\left(a_1+a_{11}\right)}{2}=11 a_6>0\),故②正確;
③\(S_{12}=6(a_1+a_{12} )=6(a_6+a_7 )>0\),故③錯誤;
④\(\because a_6>0\),\(a_7<0\),\(\therefore\)數列\(\{S_n\}\)中的最大項為\(S_6\),故④錯誤;
⑤\(\because a_6>0\),\(a_7<0\), \(a_6+a_7>0\), \(\therefore |a_6 |>|a_7 |\),故⑤正確.
綜上,①②⑤正確,故選\(A\). -
答案 \(-243\)
解析 \(\because a_{n+2}-a_n=2(n∈N^* )\),\(a_1=-23\),
\(\therefore\)數列\(\{a_n\}\)的奇數項是以\(-23\)為首項,\(2\)為公差的等差數列,
故\(a_n=-23+n-1=n-24\),
故\(n≤23\)且\(n\)為奇數時,\(a_n<0\);\(n≥25\)且\(n\)為奇數時,\(a_n>0\),
\(\because a_2=-19\),\(a_{n+2}-a_n=2(n∈N^* )\),
\(\therefore\)數列\(\{a_n\}\)的偶數項是以\(-19\)為首項,\(2\)為公差的等差數列,
故\(a_n=-19+n-2=n-21\),
故\(n≤20\)且\(n\)為偶數時,\(a_n<0\);\(n≥22\)且\(n\)為偶數時,\(a_n>0\),
且\(a_{22}=1\),\(a_{23}=-1\),\(a_{24}=3\),
故\(S_n\)的最小值為 \(S_{21}=S_{23}=-23 \times 11+\dfrac{11 \times 10}{2} \times 2-19 \times 10+\dfrac{10 \times 9}{2} \times 2=-243\).
故答案為:\(-243\). -
答案(1) \(a_n=12-n\);(2) \(66\);(3) \(\left|a_1\right|+\left|a_2\right|+\left|a_3\right|+\cdots+\left|a_n\right|=\left\{\begin{array}{l} -\dfrac{1}{2} n^2+\dfrac{23}{2} n, n \leq 12 \\ \dfrac{1}{2} n^2-\dfrac{23}{2} n+132, n>12 \end{array}\right.\).
解析 (1)\(a_2+a_6=a_3+a_5=16\),又\(a_3\cdot a_5=63\),
所以\(a_3\)與\(a_5\)是方程\(x^2-16x+63=0\)的兩根,
解得\(\left\{\begin{array}{l} a_3=7 \\ a_5=9 \end{array}\right.\)或 \(\left\{\begin{array}{l} a_3=9 \\ a_5=7 \end{array}\right.\),
又該等差數列遞減,所以\(\left\{\begin{array}{l} a_3=9 \\ a_5=7 \end{array}\right.\),
則公差\(d=\dfrac{a_5-a_3}{2}=-1\),\(a_1=11\),
所以\(a_n=11+{n-1}(-1)=12-n\);
(2)由 \(\left\{\begin{array}{l} a_n \geq 0 \\ a_{n+1} \leq 0 \end{array}\right.\),即 \(\left\{\begin{array}{l} 12-n \geq 0 \\ 11-n \leq 0 \end{array}\right.\),解得\(11≤n≤12\),
又\(n∈N^*\),所以當\(n=11\)或\(12\)時\(S_n\)取最大值,
最大值為 \(S_{11}=S_{12}=12 \times 11+\dfrac{12 \times 11}{2}(-1)=66\);
(3)由(2)知,當\(n≤12\)時\(a_n≥0\),當\(n>12\)時\(a_n<0\),
①當\(n≤12\)時,
\(|a_1 |+|a_2 |+|a_3 |+⋯+|a_n |=a_1+a_2+a_3+⋯+a_n\)
\(=S_n=\dfrac{n\left(a_1+a_n\right)}{2}=\dfrac{n(11+12-n)}{2}=-\dfrac{1}{2} n^2+\dfrac{23}{2} n\);
②當\(n>12\)時,
\(|a_1 |+|a_2 |+|a_3 |+⋯+|a_n |=(a_1+a_2+a_3+⋯+a_{12} )-(a_{13}+a_{14}+⋯+a_n)\)
\(=-S_n+2 S_{12}=\dfrac{1}{2} n^2-\dfrac{23}{2} n+2 \times 66=\dfrac{1}{2} n^2-\dfrac{23}{2} n+132\);
所以\(\left|a_1\right|+\left|a_2\right|+\left|a_3\right|+\cdots+\left|a_n\right|=\left\{\begin{array}{l} -\dfrac{1}{2} n^2+\dfrac{23}{2} n, n \leq 12 \\ \dfrac{1}{2} n^2-\dfrac{23}{2} n+132, n>12 \end{array}\right.\).
【C組---拓展題】
1.(多選)已知等差數列\(\{a_n \}\)的首項為\(a_1\),公差為\(d\),前\(n\)項和為\(S_n\),若 \(S_{20}<S_{18}<S_{19}\),則下列說法正確的是( )
A. \(a_1>0\) \(\qquad \qquad \qquad \qquad\) B. \(d>0\)
C. \(\left|a_{18}+a_{19}\right|>\left|a_{20}+a_{21}\right|\) \(\qquad \qquad \qquad \qquad\) D. 數列 \(\left\{\dfrac{S_n}{a_n}\right\}\)的所有項中最小項為 \(\dfrac{S_{20}}{a_{20}}\)
2.設無窮等差數列\(\{a_n \}\)的前\(n\)項和為\(S_n\).
(1)若首項 \(a_1=\dfrac{3}{2}\),公差\(d=1\),求滿足 \(S_{k^2}=\left(S_k\right)^2\)的正整數\(k\);
(2)求所有的無窮等差數列\(\{a_n \}\),使得對於一切正整數k都有 \(S_{k^2}=\left(S_k\right)^2\)成立.
參考答案
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答案 \(AD\)
解析 \(\because S_{20}<S_{18}<S_{19}\), \(\therefore a_{19}+a_{20}<0<a_{19}\),
\(\therefore a_{19}>0\), \(a_{20}<0\),
\(\therefore a_1+18d>0\),\(a_1+19d<0\),
\(\therefore a_1>0\),\(d<0\),
又 \(a_{19}+a_{20}<0<a_{19}\),
\(\therefore a_{21}+a_{18}<0\),
由 \(a_{20}+a_{21}-\left(a_{18}+a_{19}\right)=4 d<0\), \(a_{19}+a_{20}+a_{21}+a_{18}<0\),
\(\therefore\left|a_{20}+a_{21}\right|>\left|a_{18}+a_{19}\right|\).
由以上可得: \(a_1>a_2>\cdots>a_{19}>0>a_{20}>a_{21}>\cdots\)
\(S_{37}=\dfrac{37\left(a_1+a_{37}\right)}{2}=37 a_{19}>0\); \(S_{38}=\dfrac{38\left(a_1+a_{38}\right)}{2}=19\left(a_{19}+a_{20}\right)<0\).
\(n≤37\)時,\(S_n>0\);\(n≥38\)時,\(S_n<0\).
\(n≤19\)時,或\(n≥38\)時, \(\dfrac{S_n}{a_n}>0\);\(19<n<38\)時, \(\dfrac{S_n}{a_n}<0\).
由 \(0>a_{20}>a_{21}>\cdots>a_{37}\), \(S_{20}>S_{21}>\cdots>S_{37}>0\),
\(\therefore\)數列 \(\left\{\dfrac{S_n}{a_n}\right\}\)的所有項中最小項為 \(\dfrac{S_{20}}{a_{20}}\).
綜上可得:只有\(AD\)正確. -
答案 (1) 4;(2) ①\(a_n=0\); ②\(a_n=1\);③\(a_n=2n-1\).
解析 (1)\(\because\) 首項 \(a_1=\dfrac{3}{2}\),公差\(d=1\).
\(\therefore S_n=n a_1+\dfrac{n(n-1)}{2} d=\dfrac{3}{2} n+\dfrac{n(n-1)}{2}=\dfrac{1}{2} n^2+n\),
由 \(S_{k^2}=\left(S_k\right)^2\)得 \(\dfrac{1}{2}\left(k^2\right)^2+k^2=\left(\dfrac{1}{2} k^2+k\right)^2\),即 \(\dfrac{1}{4} k^4-k^3=0\),
\(\because k\)是正整數, \(\therefore k=4\).
(2)設數列\(\{a_n \}\)的公差為\(d\),
則在\(S_{k^2}=\left(S_k\right)^2\)中分別取\(k=1\),和\(k=2\)得 \(\left\{\begin{array}{l} S_1=\left(S_1\right)^2 \\ S_4=\left(S_2\right)^2 \end{array}\right.\),
即 \(\left\{\begin{array}{l} a_1=a_1^2,(1) \\ 4 a_1+6 d=\left(2 a_1+d\right)^2 ,(2) \end{array}\right.\)
由(1)得\(a_1=0\)或\(a_1=1\),
當\(a_1=0\)時,代入(2)得\(d=0\)或\(d=6\).若\(a_1=0\),\(d=0\)則本題成立;
若\(a_1=0\),\(d=6\),則\(a_n=6(n-1)\),
由\(S_3=18\),\((S_3 )^2=324\),\(S_9=216\)知\(S_9≠(S_3 )^2\),故所得數列不符合題意;
當\(a_1=1\)時,代入②得\(4+6d=(2+d)^2\),解得\(d=0\)或\(d=2\).
若\(a_1=1\),\(d=0\),則\(a_n=1\),\(S_n=n\)從而 \(S_{k^2}=\left(S_k\right)^2\)成立;
若\(a_1=1\),\(d=2\),則\(a_n=2n-1\),\(S_n=n^2\),
從而 \(S_{k^2}=\left(S_k\right)^2\)成立.
綜上所述,只有\(3\)個滿足條件的無窮等差數列:
①\(a_n=0\); ②\(a_n=1\);③\(a_n=2n-1\).