4.3.1 等比數列的概念1(概念、通項公式)
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基礎知識
等比數列的定義
如果一個數列從第二項起,每一項與它的前一項的比等於同一個常數,那麼這個數列叫做等比數列,
這個常數叫做等比數列的公比,記為\(q\).
代數形式: \(\dfrac{a_n}{a_{n-1}}=q\)\(( q\)是常數,\(n≥2)\)
解釋
(1) 公比是每一項與它的前一項的比,常數指的是與\(n\)無關;
(2) 等比數列中\(a_n≠0\) ,\(q≠0\)(否則數列會出現\(0\),不可能符合等邊數列定義);
(3) \(\dfrac{a_n}{a_{n-1}}=2(n \geq 2) \Rightarrow\left\{a_n\right\}\)是公比為\(2\)的等比數列;
\(\dfrac{a_{n+1}}{a_n}=-3 \Rightarrow\left\{a_n\right\}\)是公比為\(-3\)
\(\dfrac{a_{n+1}}{a_n}=4 n \Rightarrow\left\{a_n\right\}\)不是等比數列.
【例】 以下數列是等比數列的\(\underline{\quad \quad}\).
(1)\(2,,4,8,16\);\(\qquad \qquad\) (2)\(a,a,a,,a\);\(\qquad \qquad\) (3)數列\(\{a_n \}\)滿足 \(\dfrac{a_n}{a_{n-1}}=-2(n \geq 2)\).
答案 (1)(3).當\(a=0\)時,(2)不是等比數列
等比中項
若\(a\),\(b\) ,\(c\)
證明 若\(a\),\(b\) ,\(c\)成等比數列,由等比數列的定義可得\(\dfrac{b}{a}=\dfrac{c}{b}\),即\(b^2=ac\).
【例】 若\(2x\)是\(1\)與\(x+3\)的等比中項,則\(x=\)\(\underline{\quad \quad}\) .
解 依題意得\(4x^2=1\cdot (x+3)\),解得\(x=1\)或 \(-\dfrac{3}{4}\).
通項公式
等比數列\(\{a_n \}\)的首項為\(a_1\),公比為\(q\),則\(a_n=a_1 q^{n-1}\).(由定義與累乘法可得)
解釋
(1) 證明 由等比數列的定義可得, \(\dfrac{a_{n+1}}{a_n}=q\),
所以\(\dfrac{a_2}{a_1}=q\), \(\dfrac{a_3}{a_2}=q\), \(\dfrac{a_4}{a_3}=q\),\(…\)\(\dfrac{a_n}{a_{n-1}}=q(n \geq 2)\),
把以上\(n-1\)個等式累乘可得\(\dfrac{a_n}{a_1}=q^{n-1}(n \geq 2)\),即\(a_n=a_1 q^{n-1} (n≥2)\),
當\(n=1\)時,\(a_1=a_1 q^0=a_1 q^{1-1}\),即當\(n=1\)時上式也成立,
故\(a_n=a_1 q^{n-1} (n∈N^*)\).
以上的方法稱之為累乘法.
(2) 通項公式\(a_n=a_1 q^{n-1}\)告訴你:已知等比數列的首項\(a_1\)與公比\(q\)可求得任何一項
(3)等比數列的通項公式可整理為\(a_n=a_1 q^{n-1}=\dfrac{a_1}{q} \cdot q^n\),當\(q>0\),且\(q≠1\)時,可以看成\(n\)的指數函式型函式.比如等比數列\(\{2^n\}\)的各點都在指數函式\(y=2^x\)上.
(4)偶數項的正負、奇數項的正負相同(\(\dfrac{a_{2 n}}{a_{2(n-1)}}= q^2>0\),故\(a_{2n}\),\(a_{2(n-1)}\)同號,即偶數項的正負相同;奇數項同理).
【例1】 若\(-1\) ,\(b_1\) ,\(b_2\) ,\(b_3\),\(-4\)成等比數列,則\(b_2=\)\(\underline{\quad \quad}\).
解:\(b_2^2=-1×(-4)=4⇒b_2=±2\),而\(b_2=-1\cdot q^2<0\),故\(b_2=-2\).
(\(b_2\)與\(-1\) ,\(-4\)均是奇數項,符號相同\()\)
【例2】 等比數列\(\{a_n \}\)中,\(a_1=2\),\(q=3\),則\(a_n\)等於\(\underline{\quad \quad}\) .
答案 \(2×3^{n-1}\)
證明一個數列是等比數列的方法
① 定義法: \(\dfrac{a_n}{a_{n-1}}=q\)\(( q\)是常數,\(n≥2)⇒\{a_n\}\)是等比數列;
② 中項法:\(a_{n+1}^2=a_n a_{n+2} (a_n≠ 0 ,n∈ N^*)⇒\{a_n\}\)是等比數列;
③ 通項公式法:若數列的通項公式是形如\(a_n=k\cdot q^n\) \((k ,q\)是不為\(0\)常數\()\), 則數列\(\{a_n \}\)是等比數列.
基本方法
【題型1】等比數列的判定和證明
【典題1】 (多選)已知數列\(\{a_n \}\)是等比數列,那麼下列數列一定是等比數列的是( )
A.\(\left\{\dfrac{1}{a_n}\right\}\) \(\qquad \qquad \qquad\) B.\(\{ \log_2a_n \}\) \(\qquad \qquad \qquad\) C.\(\{a_n\cdot a_{n+1}\}\) \(\qquad \qquad \qquad\) D.\(\{a_n+a_{n+1}+a_{n+2}\}\)
解析 由題意,可設等比數列\(\{a_n \}\)的公比為\(q(q≠0)\),則\(a_n=a_1 q^{n-1}\).
對於\(A\): \(\dfrac{1}{a_n}=\dfrac{1}{a_1 q^{n-1}}=\dfrac{1}{a_1}\left(\dfrac{1}{q}\right)^{n-1}\).
\(\therefore\)數列 \(\left\{\dfrac{1}{a_n}\right\}\)是一個以 \(\dfrac{1}{a_1}\)為首項, \(\dfrac{1}{q}\)為公比的等比數列;
對於\(B\): \(\log _2 a_n=\log _2\left(a_1 q^{n-1}\right)=\log _2 a_1+(n-1) \log _2 q\).
\(\therefore\)數列\(\{ \log_2a_n \}\)是一個以\(\log_2a_1\)為首項,\(\log_2q\)為公差的等差數列;
對於\(C\): \(\because \dfrac{a_{n+1} \cdot a_{n+2}}{a_n \cdot a_{n+1}}=\dfrac{a_{n+2}}{a_n}=\dfrac{a_1 \cdot q^{n+1}}{a_1 \cdot q^{n-1}}=q^2\),
\(\therefore\)數列\(\{a_n a_{n+1}\}\)是一個以\(q^2\)為公比的等比數列;
對於\(D\): \(\because \dfrac{a_{n+1}+a_{n+2}+a_{n+3}}{a_n+a_{n+1}+a_{n+2}}=\dfrac{q\left(a_n+a_{n+1}+a_{n+2}\right)}{a_n+a_{n+1}+a_{n+2}}=q\),
\(\therefore\)數列\(\{a_n+a_{n+1}+a_{n+2}\}\)是一個以\(q\)為公比的等比數列.
故選:\(ACD\).
點撥 證明\(\{a_n \}\)是等比數列常見的方法是定義法: \(\dfrac{a_n}{a_{n-1}}=q\)\(( q\)是常數,\(n≥2)\);
選擇題也可採取排除法,檢驗前三項是否成等比數列.
【典題2】已知數列\(\{a_n \}\)的前\(n\)項和為\(S_n\),且滿足 \(S_n=\dfrac{3}{2} a_n+b\)\(\left(n \in N^*, b \in R, b \neq 0\right)\).
(1)求證:\(\{a_n \}\)是等比數列;\(\qquad \qquad\) (2)求證:\(\{a_n+1\}\)不是等比數列.
證明 (1)因為 \(S_n=\dfrac{3}{2} a_n+b\),所以當\(n≥2\)時 \(S_{n-1}=\dfrac{3}{2} a_{n-1}+b\),
兩式相減得 \(S_n-S_{n-1}=\dfrac{3}{2} a_n+b-\dfrac{3}{2} a_{n-1}-b\),
\(\therefore a_n=\dfrac{3}{2} a_n-\dfrac{3}{2} a_{n-1}\),
\(\therefore a_n=3a_{n-1}\),
故\(\{a_n \}\)是公比為\(q=3\)的等比數列.
(2)假設:\(\{a_n+1\}\)是等比數列,則有:\((a_n+1)^2=(a_{n+1}+1)( a_{n-1}+1)\),
即:\(a_n^2+2a_n+1=a_{n+1} a_{n-1}+a_{n+1}+a_{n-1}+1\),
由(1)知\(\{a_n \}\)是等比數列,所以\(a_n^2=a_{n+1} a_{n-1}\),
於是\(2a_n=a_{n+1}+a_{n-1}\),即\(6a_n=a_{n-1}+9a_{n-1}\),解得\(a_{n-1}=0\),
這與\(\{a_n \}\)是等比數列相矛盾,
故假設錯誤,即:\(\{a_n+1\}\)不是等比數列.
【鞏固練習】
1.根據下列通項能判斷數列為等比數列的是( )
A.\(a_n=n\) \(\qquad \qquad \qquad\) B. \(a_n=\sqrt{n}\) \(\qquad \qquad \qquad\) C.\(a_n=2^{-n}\) \(\qquad \qquad \qquad\) D.\(a_n= \log_2n\)
2.已知數列\(\{a_n \}\)是等比數列,則下列數列中:①\(\{a_n^3\}\);②\(\{2^{a_n}\}\);③ \(\left\{\dfrac{1}{2 a_n}\right\}\),等比數列的個數是( )
A.\(0\)個 \(\qquad \qquad \qquad \qquad\) B.\(1\)個 \(\qquad \qquad \qquad \qquad\) C.\(2\)個 \(\qquad \qquad \qquad \qquad\)D.\(3\)個
3.已知數列\(\{a_n \}\)滿足\(\lga_n=3n+5\),求證:\(\{a_n \}\)是等比數列.
4.已知數列\(\{a_n \}\)滿足\(a_1=1\),\(2a_{n+1}=3a_n+1\).證明:\(\{a_n+1\}\)是等比數列.
參考答案
-
答案 \(C\)
解析 在\(A\)中,\(a_n=n\)是等差數列,不是等比數列,故\(A\)錯誤;
在\(B\)中, \(a_n=\sqrt{n}\)既不是等差數列,又不是等比數列,故\(B\)錯誤;
在\(C\)中,\(a_n=2^{-n}\)是等比數列,故\(C\)正確;
在\(D\)中,\(a_n= \log_2n\)既不是等差數列,又不是等比數列,故\(D\)錯誤.
故選:\(C\). -
答案 \(C\)
解析 \(\because\)數列\(\{a_n \}\)是等比數列,設公比為\(q\),則\(q\)為常數.
則 \(a_n^3=a_1^3 q^{3 n-3}\),則 \(\dfrac{a_{n+1^3}}{a_n^3}=q^3\),為常數,故:①\(\{a_n^3\}\)為等比數列.
\(\because \dfrac{2^{a_{n+1}}}{2^{a_n}}=2^{a_{n+1}-a_n}\),不是常數,故② \(\{2^{a_n}\}\)不是等比數列.
\(\because \dfrac{\frac{1}{2 a_{n+1}}}{\frac{1}{2 a_n}}=\dfrac{a_n}{a_{n+1}}=\dfrac{1}{q},\),為常數,故③ \(\left\{\dfrac{1}{2 a_n}\right\}\)為等比數列,
故選:\(C\). -
證明 \(\because \lg a_n=3 n+5\), \(\therefore a_n=10^{3 n+5}\).
\(\therefore a_{n+1}=10^{3(n+1)+5}=10^{3 \mathrm{n}+8}\).
\(\therefore \dfrac{a_{n+1}}{a_n}=\dfrac{10^{3 n+8}}{10^{3 n+5}}=1000\).
\(\therefore\) 數列\(\{a_n \}\)是等比數列. -
證明 由\(2a_{n+1}=3a_n+1\),得\(a_{n+1}=\dfrac{3}{2} a_n+\dfrac{1}{2}\),即\(a_{n+1}+1=\dfrac{3}{2}\left(a_n+1\right)\),
故\(\dfrac{a_{n+1}+1}{a_n+1}=\dfrac{3}{2}\).
又\(a_1+1=2\),
所以\(\{a_n+1\}\)是首項為\(2\),公比為 \(\dfrac{3}{2}\)的等比數列.
【題型2】 等比數列的通項公式
【典題1】 在等比數列\(\{a_n \}\)中,\(a_5-a_1=15\),\(a_4-a_2=6\),則\(a_3=\)( )
A.\(-4\) \(\qquad \qquad \qquad \qquad\) B.\(4\) \(\qquad \qquad \qquad \qquad\) C.\(-4\)或\(4\) \(\qquad \qquad \qquad \qquad\) D.\(-8\)或\(8\)
解析 設等比數列的公比為\(q\),則
\(\because a_5-a_1=15\),\(a_4-a_2=6\),
\(\therefore\left\{\begin{array} { l }
{ a _ { 1 } q ^ { 4 } - a _ { 1 } = 1 5 } \\
{ a _ { 1 } q ^ { 3 } - a _ { 1 } q = 6 }
\end{array} \Rightarrow \left\{\begin{array} { c }
{ a _ { 1 } ( q ^ { 4 } - 1 ) = 1 5 } \\
{ a _ { 1 } ( q ^ { 3 } - q ) = 6 }
\end{array} \Rightarrow \left\{\begin{array}{c}
a_1\left(q^2-1\right)\left(q^2+1\right)=15 \\
a_1 q\left(q^2-1\right)=6
\end{array}\right.\right.\right.\),
\(\therefore q^2+1=\dfrac{5}{2} q\), \(\therefore q=2\)或 \(q=\dfrac{1}{2}\),
\(\therefore a_1=1\)或\(a_1=-16\),\(\therefore a_3=±4\),
故選:\(C\).
點撥 本題採取基本量法,\(a_1\),\(q\)是等比數列的基本量,在等比數列中遇到\(a_n\)採取通項公式 \(a_n=a_1 \mathrm{q}^{\mathrm{n}-1}\);由通項公式由已知條件得到關於\(a_1\),\(q\)的方程組,再通過因式分解消元解方程.
【典題2】已知數列\(\{a_n \}\)中,\(a_1=7\),\(a_3=1\),若 \(\left\{\dfrac{1}{a_n+1}\right\}\)是等比數列,則\(a_{11}\)等於( )
A.\(-\dfrac{31}{32}\) \(\qquad \qquad \qquad \qquad\) B.\(-\dfrac{63}{64}\) \(\qquad \qquad \qquad \qquad\) C.\(-\dfrac{127}{128}\) \(\qquad \qquad \qquad \qquad\) D.\(-\dfrac{255}{256}\)
解析 設等比數列 \(\left\{\dfrac{1}{a_n+1}\right\}\)的公比為\(q\),\(a_1=7\),\(a_3=1\),
則 \(\dfrac{1}{1+1}=\dfrac{1}{1+7} \cdot q^2\),解得\(q^2=4\).
\(\therefore \dfrac{1}{a_{11}+1}=\dfrac{1}{a_1+1} \cdot q^{10}=\dfrac{1}{8} \times 2^{10}=2^7=128\),解得\(a_{11}=-\dfrac{127}{128}\),
故選:\(C\).
【鞏固練習】
1.已知數列\(\{a_n \}\)滿足\(a_1=1\),\(a_{n+1}=2a_n (n∈N^* )\),則\(a_4=\)( )
A.\(4\) \(\qquad \qquad \qquad \qquad\) B.\(6\) \(\qquad \qquad \qquad \qquad\) C. \(8\) \(\qquad \qquad \qquad \qquad\) D.\(2\)
2.設\(\{a_n \}\)是等比數列,下列結論中不正確的是( )
A.若\(a_1 a_2>0\),則\(a_2 a_3>0\) \(\qquad \qquad \qquad \qquad\) B.若\(a_1+a_3<0\),則\(a_5<0\) \(\qquad \qquad \qquad \qquad\)
C.若\(a_1 a_2<0\),則\(a_1 a_5<0\) \(\qquad \qquad \qquad \qquad\) D.若\(0<a_1<a_2\),則\(a_1+a_3>2a_2\)
3.已知\(-1\),\(a_1,\)\(a_2\),\(-4\)成等差數列,且\(-1\),\(b_1\),\(b_2\),\(b_3\),\(-4\)成等比數列,則 \(\dfrac{a_1+a_2}{b_2}\) 的值為\(\underline{\quad \quad}\).
4.\(\{a_n \}\)是各項均為正數的等差數列,\(\{b_n\}\)是等比數列,已知\(\dfrac{a_1}{b_1}=\dfrac{a_2}{b_2}=1\), \(\dfrac{a_3}{b_3}=\dfrac{8}{9}\),那麼 \(\dfrac{a_4}{b_4}=\)\(\underline{\quad \quad}\).
參考答案
-
答案 \(C\)
解析 因為\(a_1=1\),\(a_{n+1}=2a_n (n∈N^* )\),
所以 \(\dfrac{a_n+1}{a_n}=2\),即數列\(\{a_n \}\)是以\(1\)為首項,\(2\)為公比的等比數列,
所以\(a_n=2^{n-1}\),可得\(a_4=2^3=8\).
故選:\(C\). -
答案 \(C\)
解析 設等比數列\(\{a_n \}\)的公比為\(q\),
\(A\).\(\because a_1 a_2>0\),\(\therefore a_1^2 q>0\),\(\therefore q>0\),則\(a_2 a_3=a_1^2 q^3>0\),正確.
\(B\).\(\because a_1+a_3<0\),\(\therefore a_1 (1+q^2)<0\),\(\therefore a_1<0\),則\(a_5=a_1 q^4<0\),正確.
\(C\).\(\because a_1 a_2<0\),\(\therefore a_1^2 q<0\),\(\therefore q<0\),則\(a_1 a_5=a_1^2 q^4>0\),因此不正確.
\(D\).\(\because 0<a_1<a_2\),\(\therefore 0<a_1<a_1 q\),\(\therefore a_1>0\),\(q>0\),\(q≠1\).
則\(a_1+a_3=a_1 (1+q^2)>2a_1 q=2a_2\),正確.
故選:\(C\). -
答案 \(\dfrac{5}{2}\)
解析 以題意得\(a_1+a_2=-1+(-4)=-5\),\(b^2=-1×(-4)=4⇒b=±2\),
因\(-1\),\(b_1\),\(b_2\)成等比數列,\(\therefore b_1^2=-b_2>0⇒b_2<0\),
\(\therefore b_2=-2\),則 \(\dfrac{a_1+a_2}{b_2}=\dfrac{5}{2}\). -
答案 \(\dfrac{20}{27}\)
解析 設等差數列\(\{a_n \}\)的公差為\(d\),等比數列\(\{b_n\}\)的公比為\(q\),
則\(a_1+d=a_1 q\),\(9(a_1+2d)=8a_1 q^2\),
聯立可得\(8q^2-18q+9=0\),解得: \(q=\dfrac{3}{2}\)或 \(q=\dfrac{3}{4}\).
\(\because \{a_n\}\)是各項均為正數,則\(d>0\),\(\therefore q>1\),則 \(q=\dfrac{3}{2}\),
\(\therefore b_4=a_1 \times\left(\dfrac{3}{2}\right)^3=\dfrac{27}{8} a_1\), \(\therefore \dfrac{a_2}{b_2}=\dfrac{3}{2} a_1\),
則\(d=\dfrac{3}{2} a_1-a_1=\dfrac{1}{2} a_1\),
\(\therefore a_4=a_1+3 d=\dfrac{5}{2} a\), \(\therefore \dfrac{a_4}{b_4}=\dfrac{\frac{5}{2} a_1}{\frac{27}{8} a_1}=\dfrac{20}{27}\).
【題型3】應用問題
【典題1】 某工廠\(2010\)年\(1\)月的生產總值為\(a\)萬元,計劃從\(2010\)年\(2\)月起,每個月生產總值比上一個月增長\(m\%\),那麼到\(2011\)年\(8\)月底該廠的生產總值為多少萬元?
解析 設從\(2010\)年\(1\)月開始,第\(n\)個月該廠的生產總值是\(a_n\)萬元,則\(a_{n+1}=a_n+a_n m\%\),
\(\therefore \dfrac{a_{n+1}}{a_n}= =1+m\%\),
\(\therefore\)數列\(\{a_n \}\)是首項\(a_1=a\),公比\(q=1+m\%\)的等比數列.
\(\therefore a_n=a\left(1+m\%\right)^{n-1}\).
\(\therefore 2011\)年\(8\)月底該廠的生產總值為 \(a_{20}=a(1+m \%)^{20-1}=a(1+m \%)^{19}\) (萬元).
【典題2】已知數列\(\{a_n \}\)滿足\(S_n=n-a_n\).
(1)求證:數列\(\{a_n-1\}\)是等比數列;\(\qquad \qquad\) (2)求\(a_n\).
解析 (1)證明:\(\because\) 數列\(\{a_n \}\)滿足\(S_n=n-a_n\).
\(\therefore S_{n+1}=n+1-a_{n+1}\),兩式相減可得\(S_{n+1}-S_n={n+1}-n-a_{n+1}+a_n\),
\(\therefore a_{n+1}=1-a_{n+1}+a_n\), \(\therefore a_{n+1}=\dfrac{1}{2}+\dfrac{1}{2} a_n,\),
\(\therefore \dfrac{a_{n+1}-1}{a_n-1}=\dfrac{\dfrac{1}{2}+\dfrac{1}{2} a_n-1}{a_n-1}=\dfrac{\dfrac{1}{2}\left(a_n-1\right)}{a_n-1}=\dfrac{1}{2}\),
\(\therefore\) 數列\(\{a_n-1\}\)是\(\dfrac{1}{2}\)為公比的等比數列;
(2)由(1)可得數列\(\{a_n-1\}\)是\(\dfrac{1}{2}\)為公比的等比數列,
由\(S_n=n-a_n\)可得\(a_1=S_1=1-a_1\),解得\(a_1=\dfrac{1}{2}\),
故\(a_1-1=-\dfrac{1}{2}\),
\(\therefore a_n-1=\dfrac{1}{2}×(\dfrac{1}{2})^{n-1}=(\dfrac{1}{2})^n\),
\(\therefore a_n=1+(\dfrac{1}{2})^n\).
【鞏固練習】
1.河南洛陽龍門石窟是中國石刻藝術寶庫,現為世界非物質文化遺產之一.某洞窟的浮雕共\(8\)層,它們構成一幅優美的圖案.各層浮雕數成等比數列,第二層浮雕數為\(6\),第\(5\)層浮雕數為\(48\),則第\(7\)層浮雕數為( )
A.\(96\) \(\qquad \qquad \qquad \qquad\) B.\(128\) \(\qquad \qquad \qquad \qquad\) C. \(192\) \(\qquad \qquad \qquad \qquad\) D.\(384\)
2.我國古代數學著作《九章算術》中有“竹九節”問題:現有一根\(9\)節的竹子,自上而下各節的容積成等比數列,最上面\(3\)節的容積之積為\(3\)升,最下面\(3\)節的容積之積為\(243\)升,則第\(5\)節的容積是\(\underline{\quad \quad}\)升.
3.已知等比數列\(\{a_n \}\)的各項均為正數,且\(a_1>1\),前\(n\)項之積為\(T_n\),設 \(T_{10}=T_{20}\),
(1)當\(n\)為何值時,\(T_n\)最大?
(2)是否存在自然數\(n\),使得\(T_n=1\)?
4.在數列\(\{a_n \}\)中.已知\(a_1=2\), \(a_{n+1}=\dfrac{2 a_n}{a_n+1}\).
(1)求證: \(\left\{\dfrac{1}{a_n}-1\right\}\)是等比數列,
(2)若對任意\(n∈N_+\),\(a_n>m\)恆成立,求\(m\)的最大值.
參考答案
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答案 \(C\)
解析 設第\(2\)層浮雕數為\(a_2\),第\(5\)層浮雕數為\(a_5\),公比為\(q\),
則\(a_2=6\),\(a_5=48\),
\(\because\)各層浮雕數成等比數列,
\(\therefore 48=6×q^3\),\(\therefore q=2\),
\(\therefore\) 第\(7\)層浮雕數為\(a_7=a_5⋅q^2=48×4=192\),
故選:\(C\). -
答案 \(3\)
解析 設第\(n(n≤9,n∈N^* )\)節的容積為\(a_n\),則\(\{a_n \}\)是等比數列,
\(\because\)最上面\(3\)節的容積之積為\(3\)升,最下面\(3\)節的容積之積為\(243\)升,
\(\therefore\left\{\begin{array}{l} a_1 \cdot a_1 q^{\prime} a_1 q^2=3 \\ a_1 q^6 \cdot a_1 q^7 \cdot a_1 q^8=243 \end{array}\right.\),解得 \(a_1 q=3^{\dfrac{1}{3}}\), \(q^3=9^{\dfrac{1}{3}}\),
\(\therefore\)第\(5\)節的容積 \(a_5=a_1 q^4=a_1 q \cdot q^3=3^{\dfrac{1}{3}} \cdot 9^{\dfrac{1}{3}}=3\)(升). -
答案 (1) 當\(n=10\)時,\(T_n\)最大 ;(2) 當\(n=20\)時,\(T_{20}=1\).
解析 (1)設等比數列\(\{a_n \}\)的公比為\(q>0\),且\(a_1>1\),\(\because T_{10}=T_{20}\),
\(\therefore a_{11} a_{12} \cdot \ldots \cdot a_{20}=1\),
\(\therefore a_1^{10} q^{10+11+\cdots+19}=a_1^{10} q^{145}=1\),
\(\therefore a_1^2 q^{19}=1\).
\(\therefore T_n=a_1^n \cdot q^{1+2+\cdots+(n-1)}=a_1^n q^{\dfrac{n(n-1)}{2}}=a_1^{\dfrac{n(20-n)}{19}}\).
可知:當\(n=10\)時,指數\(\dfrac{n(20-n)}{19}\)取得最大值 \(\dfrac{100}{19}\),
\(\therefore\) 當\(n=10\)時,\(T_n\)最大.
(2)由 \(T_n=a_1 \dfrac{n(20-n)}{19}\),可知:當\(n=20\)時,\(T_{20}=1\). -
答案 (1)略 ;(2) \(1\) .
解析 證明:(1)\(\because a_1=2\), \(a_{n+1}=\dfrac{2 a_n}{a_n+1}\),
\(\therefore a_n>0\)恆成立;\(a_{n+1} a_n+a_{n+1}=2a_n\),
\(\therefore 1+\dfrac{1}{a_n}=2 \dfrac{1}{a_{n+1}}\) ,
\(\therefore \dfrac{1}{a_n}-1=2\left(\dfrac{1}{a_{n+1}}-1\right)\),
\(\therefore \dfrac{1}{a_{n+1}}-1=\dfrac{1}{2}\left(\dfrac{1}{a_n}-1\right)\),
又 \(\because \dfrac{1}{a_1}-1=-\dfrac{1}{2}\),
\(\therefore\left\{\dfrac{1}{a_n}-1\right\}\)是以\(-\dfrac{1}{2}\)為首項,\(\dfrac{1}{2}\)為公比的等比數列;
(2) \(\therefore\left\{\dfrac{1}{a_n}-1\right\}\)是以\(-\dfrac{1}{2}\)為首項,\(\dfrac{1}{2}\)為公比的等比數列,
\(\therefore \dfrac{1}{a_n}-1=-\dfrac{1}{2} \cdot\left(\dfrac{1}{2}\right)^{n-1}=-\left(\dfrac{1}{2}\right)^n\),
\(\therefore a_n=\dfrac{2^n}{2^n-1}=1+\dfrac{1}{2^n-1}\),
\(\therefore\)數列\(\{a_n \}\)是遞減數列,且當\(n→+∞\)時,\(a_n→1\);
\(\therefore a_n>1\)恆成立,
\(\therefore m\)的最大值為\(1\).
分層練習
【A組---基礎題】
1.在等比數列\(\{a_n \}\)中,已知\(a_1=\dfrac{9}{8}\),\(a_n=\dfrac{1}{3}\),\(q=\dfrac{2}{3}\),則\(n\)的值為( )
A.\(3\) \(\qquad \qquad \qquad \qquad\) B.\(4\) \(\qquad \qquad \qquad \qquad\) C. \(5\) \(\qquad \qquad \qquad \qquad\) D.\(6\)
2.在等比數列\(\{a_n \}\)中,已知\(a_1=2\),\(a_1-a_3+a_5=26\),則\(a_3=\)( )
A.\(20\) \(\qquad \qquad \qquad \qquad\) B.\(12\) \(\qquad \qquad \qquad \qquad\) C. \(8\) \(\qquad \qquad \qquad \qquad\) D.\(4\)
3.已知數列\(\{a_n \}\)是等比數列,下列四個命題中不正確的命題有( )
A.數列\(\{|a_n |\}\)是等比數列 \(\qquad \qquad \qquad \qquad\) B.數列\(\{a_n a_{n+1}\}\)是等比數列
C.數列 \(\left\{\dfrac{1}{a_n}\right\}\)是等比數列 \(\qquad \qquad \qquad \qquad\) D.數列\(\{\lg a_n^2\}\)是等比數列
4.標準對數視力表採用的“五分記錄法”是我國獨創的視力記錄方式,此表由\(14\)行開口方向各異的正方形\(“E”\)形視標所組成,從上到下分別對應視力\(4.0,4.1,…,5.2,5.3\),且從第一行開始往下,每一行\(“E”\)形視標邊長都是下一行\(“E”\)形視標邊長的\(\sqrt[10]{10}\)倍,若視力\(4.1\)的視標邊長為\(a\),則視力\(4.9\)的視標邊長為( )
A. \(10^{\frac{2}{5}} a\) \(\qquad \qquad \qquad \qquad\) B. \(10^{-\frac{2}{5}} a\)\(\qquad \qquad \qquad \qquad\) C. \(10^{\frac{4}{5}} a\) \(\qquad \qquad \qquad \qquad\) D. \(10^{-\frac{4}{5}} a\)
5.(多選)已知等比數列\(\{a_n \}\)中,滿足\(a_1=1\),公比\(q=-2\),則( )
A.數列\(\{2a_n+a_{n+1}\}\)是等比數列 \(\qquad \qquad \qquad \quad\) B.數列\(\{a_{n+1}-a_n\}\)是等比數列
C.數列\(\{a_n a_{n+1}\}\)是等比數列 \(\qquad \qquad \qquad \qquad\) D.數列\(\{ \log_2|a_n |\}\)是遞減數列
6.一批裝置價值\(a\)萬元,由於使用磨損,每年比上一年價值降低\(b\%\),則\(n\)年後這批裝置的價值為\(\underline{\quad \quad}\) .
7.設公差不為零的等差數列\(\{a_n \}\)滿足\(a_3=7\),且\(a_1-1\),\(a_2-1\),\(a_4-1\)成等比數列,則\(a_{10}\)等於 .
8.\(△ABC\),若\(\sin A\),\(\cos \dfrac{B}{2}\),\(\sin C\)成等比數列,則\(△ABC\)的形狀為\(\underline{\quad \quad}\).
9.已知數列\(\{a_n \}\)中,\(a_1=1\),\(a_{n+1}=2a_n+1\),\((n∈N^*)\).求證:數列\(\{a_n+1\}\)是等比數列.
10.已知等比數列\(\{a_n \}\), \(a_1 a_2=-\dfrac{1}{2}\), \(a_3=\dfrac{1}{4}\).
(1)求數列\(\{a_n \}\)的通項公式;
(2)證明:對任意\(k∈N^*\),\(a_k\),\(a_{k+2}\), \(a_{k+1}\)成等差數列.
11.數列\(\{a_n \}\)滿足 \(a_{n+1}=\dfrac{1}{2} a_n+1\),\(a_1=1\),若\(b_n=a_n-2\).
(1)求證:數列\(\{b_n \}\)是等比數列;(2)求數列\(\{a_n \}\)的通項公式.
參考答案
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答案 \(B\)
解析 \(\because\)在等比數列\(\{a_n \}\)中,已知 \(a_1=\dfrac{9}{8}\), \(a_n=\dfrac{1}{3}\), \(q=\dfrac{2}{3}\),
\(\therefore a_n=\dfrac{9}{8} \times\left(\dfrac{2}{3}\right)^{n-1}=\dfrac{1}{3}\), \(\therefore\left(\dfrac{2}{3}\right)^{n-1}=\dfrac{8}{27}=\left(\dfrac{2}{3}\right)^3\),
\(\therefore n-1=3\),可得\(n=4\),
故選:\(B\). -
答案 \(C\)
解析 根據題意,設等比數列\(\{a_n \}\)的公比為\(q\),
已知\(a_1=2\),\(a_1-a_3+a_5=26\),
則有\(a_1-a_3+a_5=a_1-a_1 q^2+a_1 q^4=2(1-q^2+q^4 )=26\),
解得\(q^2=4\)或\(-3\)(舍),
所以\(a_3=a_1 q^2=8\),
故選:\(C\). -
答案 \(D\)
解析 由\(\{a_n \}\)是等比數列可得 \(\dfrac{a_n}{a_{n-1}}=q\)\((q\)為常數,\(q≠0)\),
\(\dfrac{\left|a_n\right|}{\left|a_{n-1}\right|}=\left|\dfrac{a_n}{a_{n-1}}\right|=|q|\)為常數,故是等比數列;\(A\)正確.
\(\dfrac{a_n a_{n+1}}{a_{n-1} a_n}=\dfrac{a_{n+1}}{a_{n-1}}=q^2\)為常數,故是等比數列;\(B\)正確.
\(\dfrac{\dfrac{1}{a_n}}{\dfrac{1}{a_{n-1}}}=\dfrac{a_{n-1}}{a_n}=\dfrac{1}{q}\)常數,故是等比數列;\(C\)正確.
數列\(a_n=1\)是等比數列,但是\(\lg a_n^2=0\)不是等比數列;\(D\)不正確.
故選:\(D\). -
答案 \(D\)
解析 根據題意可知視際邊長從上到下是以 \(\sqrt[10]{10}\)為公比的等比數列,
記視力\(4.1\)的視標邊長為\(a_1=a\),
則視力\(4.9\)的視標邊長為 \(a_9=a \cdot\left(10^{-\dfrac{1}{10}}\right)^8=10^{-\dfrac{4}{5}} a\).
故選:\(D\). -
答案 \(BC\)
解析 \(\because\) 等比數列\(\{a_n \}\)中,滿足\(a_1=1\),公比\(q=-2\),
\(\therefore a_n=1×(-2)^{n-1}=(-2)^{n-1}\).
由此可得\(2a_n+a_{n+1}=2(-2)^{n-1}+(-2)^n=0\),故\(A\)錯誤;
\(a_{n+1}-a_n=(-2)^n-(-2)^{n-1}=-3(-2)^{n-1}\),
故數列\(\{a_{n+1}-a_n\}\)是等比數列,故\(B\)正確;
\(a_n a_{n+1}=(-2)^{n-1} (-2)^n=(-2)^{2n-1}\),故數列\(\{a_n a_{n+1}\}\)是等比數列,故\(C\)正確;
\(\log _2\left|a_n\right|=\log _2 2^{n-1}=n-1\),故數列\(\{ \log_2|a_n | \}\)是遞增數列,故\(D\)錯誤,
故選:\(BC\). -
答案 \(a(1-b\%)^n\)
解析 依題意可知第一年後的價值為\(a(1-b\%)\),第二年價值為\(a(1-b\%)^2\),
依此類推可知每年的價值成等比數列,其首項\(a(1-b\%)\),公比為\(1-b\%\),
進而可知\(n\)年後這批裝置的價值為\(a(1-b\%)^n\). -
答案 \(21\)
解析 設等差數列\(\{a_n \}\)的公差為\(d\),則\(d≠0\),
則\(a_1=a_3-2d=7-2d\),\(a_2=a_3-d=7-d\),\(a_4=a_3+d=7+d\),
由於\(a_1-1\),\(a_2-1\),\(a_4-1\)成等比數列,則\((a_2-1)^2=(a_1-1)(a_4-1)\),
即\((6-d)^2=(6-2d)(6+d)\),化簡得\(d^2-2d=0\),由於\(d≠0\),解得\(d=2\),
因此\(a_{10}=a_3+7d=7+7×2=21\).
故答案為:\(21\). -
答案 等腰三角形
解析 \(\because \sin A\),\(\cos \dfrac{B}{2}\),\(\sin C\)成等比數列, \(\therefore \cos ^2 \dfrac{B}{2}=\sin A \sin C\),
\(\therefore \dfrac{1}{2}(1+\cos B)=-\dfrac{1}{2}[\cos (A+C)-\cos (A-C)]\),
\(\therefore 1+\cos B=-[-\cos B-\cos (A-C)]\),
化為 \(\cos (A-C)=1\),又\(A\),\(C∈(0,π)\),
\(\therefore A=C\),可得\(a=c\).
則\(△ABC\)的形狀為等腰三角形. -
證明 \(\because a_{n+1}=2a_n+1\),\((n∈N^*)\),
\(\therefore a_{n+1}+1=2(a_n+1)\),
\(\therefore \dfrac{a_{n+1}+1}{a_n+1}=2\),
\(\therefore\)數列\(\{a_n+1\}\)是以\(2\)為公比的等比數列. -
答案 (1) \(a_n=\left(-\dfrac{1}{2}\right)^{n-1}\);(2) 略.
解析 (1)根據題意,設等比數列\(\{a_n \}\)的公比為\(q\),則\(a_n=a_1 q^{n-1}\),
若\(a_1 a_2=-\dfrac{1}{2}\),則 \(a_1^2 q=-\dfrac{1}{2}\),
若\(a_3=\dfrac{1}{4}\),則\(a_1 q^2=\dfrac{1}{4}\),變形可得\(\dfrac{a_1}{q}=-2\),解可得\(a_1^3=1\),則\(a_1=1\),
則有\(q=-\dfrac{1}{2}\) ;故\(a_n=\left(-\dfrac{1}{2}\right)^{n-1}\);
(2)證明:根據題意, \(a_n=\left(-\dfrac{1}{2}\right)^{n-1}\),
則\(a_k=\left(-\dfrac{1}{2}\right)^{k-1}\),\(a_{k+1}=\left(-\dfrac{1}{2}\right)^k\),\(a_{k+2}=\left(-\dfrac{1}{2}\right)^{k+1}\);
則\(a_k+a_{k+1}-2 a_{k+2}=\left(-\dfrac{1}{2}\right)^{k-1}+\left(-\dfrac{1}{2}\right)^k-2\left(-\dfrac{1}{2}\right)^{k+1}\)\(=\left(-\dfrac{1}{2}\right)^k\left[-2+1-2 x\left(-\dfrac{1}{2}\right)\right]=0\),則有\(a_k+a_{k+1}=2 a_{k+2}\)
故\(a_k\),\(a_{k+2}\), \(a_{k+1}\)成等差數列. -
答案 (1)略 ;(2) \(a_n=2-\dfrac{1}{2^{n-1}}\).
解析 (1)證明: \(\because a_{n+1}=\dfrac{1}{2} a_n+1,\),
\(\therefore a_n-2=\dfrac{1}{2}\left(a_{n-1}-2\right)\),又\(b_n=a_n-2\),
\(\therefore b_n=\dfrac{1}{2} b_{n-1}\),
\(\therefore \{b_n \}\)是公式為\(\dfrac{1}{2}\)的等比數列;
(2)解:\(b_1=a_1-2=-1\), \(b_n=(-1) \times\left(\dfrac{1}{2}\right)^{n-1}\),
即\(a_n-2=-\left(\dfrac{1}{2}\right)^{n-1}\), \(\therefore a_n=2-\dfrac{1}{2^{n-1}}\) .
【B組---提高題】
1.已知實數\(a>0\),\(b>0\),\(\sqrt{2}\)是\(8^a\)與\(2^b\)的等比中項,則\(\dfrac{6}{a}+\dfrac{2}{b}\)的最小值是\(\underline{\quad \quad}\).
2.設數列\(\{a_n \}\)的首項\(a_1\)為常數,且\(a_n=3^{n-1}-2a_{n-1} (n≥2)\).
(1) 判斷數列\(\left\{a_n-\dfrac{3^n}{5}\right\}\)是否為等比數列,請說明理由;
(2) \(S_n\)是數列\(\{a_n \}\)的前\(n\)項的和,若\(\{S_n\}\)是遞增數列,求\(a_1\)的取值範圍.
參考答案
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答案 \(32\)
解析 因為\(\sqrt{2}\)是\(8^a\)與\(2^b\)的等比中項,所以 \(8^a 2^b=(\sqrt{2})^2=2\),
即\(2^{3a+b}=2\),所以\(3a+b=1\),
所以\(\dfrac{6}{a}+\dfrac{2}{b}=\left(\dfrac{6}{a}+\dfrac{2}{b}\right)(3 a+b)=20+\dfrac{6 b}{a}+\dfrac{6 a}{b} \geq 20+2 \sqrt{\dfrac{6 b}{a} \cdot \dfrac{6 a}{b}}=32\),
當且僅當\(\dfrac{6 b}{a}=\dfrac{6 a}{b}\),即\(a^2=b^2\),\(a=b\)時,等號成立.
所以\(\dfrac{6}{a}+\dfrac{2}{b}\)的最小值是\(32\).
故答案為:\(32\). -
答案 (1) 略;(2) \(-\dfrac{3}{4}<a_1<\dfrac{3}{2}\) .
解析 (1)當\(n≥2\)時, \(a_n-\dfrac{3^n}{5}=3^{n-1}-2 a_{n-1}-\dfrac{3^n}{5}=2 \cdot \dfrac{3^{n-1}}{5}-2 a_{n-1}=-2\left(a_{n-1}-\dfrac{3^{n-1}}{5}\right)\)
(定義法證明等比數列,要注意首項\(a_1-\dfrac{3}{5}\)是否等於\(0\))
當\(a_1-\dfrac{3}{5} \neq 0\),即\(a_1 \neq \dfrac{3}{5}\)時,\(\dfrac{a_n-\dfrac{3^n}{5}}{a_{n-1}-\dfrac{3^{n-1}}{5}}=-2\)
\(\therefore a_1 \neq \dfrac{3}{5}\)時, \(\left\{a_n-\dfrac{3^n}{5}\right\}\)為等比數列,公比為\(-2\).
當\(a_1-\dfrac{3}{5}=0\),即 \(a_1=\dfrac{3}{5}\)時,數列\(\left\{a_n-\dfrac{3^n}{5}\right\}\)不是等比數列.
(2) \(a_1=\dfrac{3}{5}\)時, \(a_n=\dfrac{3^n}{5}\),為單調遞增數列,滿足條件.
\(a_1 \neq \dfrac{3}{5}\)時,由(1)可得: \(a_n-\dfrac{3^n}{5}=\left(a_1-\dfrac{3}{5}\right)(-2)^{n-1}\),
若\(\{S_n\}\)是遞增數列,則\(S_n-S_{n-1}>0(n≥2)\),即\(a_n>0(n≥2\)),
\(\therefore a_n=\left(a_1-\dfrac{3}{5}\right)(-2)^{n-1}+\dfrac{3^n}{5}>0\),
(問題變成恆成立問題,可想到分離引數法,遇到 \((-2)^{n-1}\)想到分\(n\)奇偶數討論)
當\(n\)為偶數,則 \(-2^n\left(a_1-\dfrac{3}{5}\right)+\dfrac{3^n}{5}>0 \Rightarrow a_1<\dfrac{2}{5} \cdot\left(\dfrac{3}{2}\right)^n+\dfrac{3}{5}\),
故\(a_1<\dfrac{2}{5} \cdot\left(\dfrac{3}{2}\right)^2+\dfrac{3}{5}=\dfrac{3}{2}\),( \(f(n)=\dfrac{2}{5} \cdot\left(\dfrac{3}{2}\right)^n+\dfrac{3}{5}\)是增數列, \(f_{\min }=f(2)\))
當\(n\)為奇數,則 \(2^n\left(a_1-\dfrac{3}{5}\right)+\dfrac{3^n}{5}>0 \Rightarrow a_1<-\dfrac{2}{5} \cdot\left(\dfrac{3}{2}\right)^n+\dfrac{3}{5}\),
故 \(a_1>-\dfrac{2}{5} \cdot\left(\dfrac{3}{2}\right)^3+\dfrac{3}{5}=-\dfrac{3}{4}\),( \(f(n)=-\dfrac{2}{5} \cdot\left(\dfrac{3}{2}\right)^n+\dfrac{3}{5}\)是減數列, \(f_{\max }=f(3)\))
\(\therefore-\dfrac{3}{4}<a_1<\dfrac{3}{2}\).且 \(a_1 \neq \dfrac{3}{5}\).
綜上可得: \(-\dfrac{3}{4}<a_1<\dfrac{3}{2}\).
【C組---拓展題】
1.數字\(1,2,3,…,n(n≥2)\)的任意一個排列記作\((a_1,a_2,…,a_n)\),設\(S_n\)為所有這樣的排列構成的集合.集合\(A_n=\{(a_1,a_2,…,a_n)∈S_n |\)任意整數\(i\),\(j\),\(1≤i<j≤n\),都有\(a_i-i≤a_j-j\}\);集合\(B_n=\{(a_1,a_2,…,a_n\}∈S_n |\)任意整數\(i\),\(j\),\(1≤i<j≤n\),都有\(a_i+i≤a_j+j\}\).
(1)用列舉法表示集合\(A_3\),\(B_3\)
(2)求集合\(A_n∩B_n\)的元素個數;
(3)記集合\(B_n\)的元素個數為\(b_n\).證明:數列\(\{b_n\}\)是等比數列.
參考答案
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答案 (1)\(A_3=\{(1,2,3)\}\),\(B_3=\{(1,2,3),(1,3,2),(2,1,3),(3,2,1)\}\) ;
(2) \(1\) ;(3) 略.
解析 (1) (列出\(S_n\)所有項逐一檢驗,對於集合\(A_n\)可理解\(f(n)=a_n-n\)不是減數列,
對於集合\(B_n\)可理解\(f(n)=a_n+n\)不是減數列)
\(A_3=\{(1,2,3)\}\),\(B_3=\{(1,2,3),(1,3,2),(2,1,3),(3,2,1)\}\).
(2)考慮集合\(A_n\)中的元素\((a_1,a_2,…,a_n)\).
由已知,對任意整數\(1≤i<j≤n\),都有\(a_i-i≤a_j-j\),
所以\((a_i-i)+i<(a_j-j)+j\),
所以\(a_i<a_j\).
由\(i\),\(j\)的任意性可知,\((a_1,a_2,…,a_n)\)是\(1,2,3,…,n\)的單調遞增排列,
所以\(A_n=\{(1,2,3,…,n)\}\).
又因為當\(a_k=k(k∈N^*,1≤k≤n)\)時,對任意整數\(i\),\(j\),\(1≤i<j≤n\),
都有\(a_i+i≤a_j+j\).
所以\((1,2,3,…,n)∈B_n\),所以\(A_n⊆B_n\).
所以集合\(A_n∩B_n\)的元素個數為\(1\).
(3)由(2)知,\(b_n≠0\).
因為\(B_2=\{(1,2),(2,1)\}\),所以\(b_2=2\).
當\(n≥3\)時,考慮\(B_n\)中的元素\((a_1,a_2,…,a_n)\).
(討論\(n\)在\(B_n\)中的位置)
(1)假設\(a_k=n(1≤k<n)\).由已知,\(a_k+k≤a_{k+1}+(k+1)\),
所以\(a_{k+1}≥a_k+k-(k+1)=n-1\),
又因為\(a_{k+1}≤n-1\),所以\(a_{k+1}=n-1\).
依此類推,若\(a_k=n\),則\(a_{k+1}=n-1\),\(a_{k+2}=n-2\),…,\(a_n=k\).
①若\(k=1\),則滿足條件的\(1,2,3,…,n\)的排列\((a_1,a_2,…,a_n)\)有\(1\)個.
②若\(k=2\),則\(a_2=n\),\(a_3=n-1\),\(a_4=n-2\),…,\(a_n=2\).
所以\(a_1=1\).
此時滿足條件的\(1,2,3,…,n\)的排列\((a_1,a_2,…,a_n)\)有\(1\)個.
③若\(2<k<n\),
只要\((a_1,a_2,a_3,…a_{k-1})\)是\(1,2,3,…,k-1\)的滿足條件的一個排列,就可以相應得到\(1,2,3,…,n\)的一個滿足條件的排列.
(把問題轉化為\(n=k-1\)的情況,這方法巧妙,可得到\(b_n\)的一遞推公式)
此時,滿足條件的\(1,2,3,…,n\)的排列\((a_1,a_2,…,a_n)\)有\(b_{k-1}\)個.
(2)假設\(a_n=n\),只需\((a_1,a_2,a_3,…a_{n-1})\)是\(1,2,3,…,n-1\)的滿足條件的排列,此時滿足條件的\(1,2,3,…,n\)的排列\((a_1,a_2,…,a_n)\)有\(b_{n-1}\)個.
綜上\(b_n=1+1+b_2+b_3+⋯+b_{n-1}\),\(n≥3\).
因為\(b_3=1+1+b_2=4=2b_2\),
且當\(n≥4\)時,\(b_n=(1+1+b_2+b_3+⋯+b_{n-2})+ b_{n-1}=2b_{n-1}\),
所以對任意\(n∈N^*\),\(n≥3\),都有 \(\dfrac{b_n}{b_{n-1}}=2\).
所以\(\{b_n\}\)成等比數列.