4.3.1 等比數列的概念2(性質運用)
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基礎知識
等比數列的基本性質
設\(\{a_n \}\)是首項為\(a_1\), 公比為\(q\)的等比數列,其中\(m\) ,\(n\) ,\(p\) ,\(t∈N^*\),那麼
(1) \(a_n=a_m q^{n-m}\);
證明 由等比數列通項公式可得\(a_n=a_1\cdot q^{n-1}\)
\(\qquad\) 兩式相除可得 \(\dfrac{a_n}{a_m}=q^{n-m}\),即\(a_n=a_m q^{n-m}\).
意義 求等比數列任一項\(a_k\)或通項公式\(a_n\),不一定要求\(a_1\),可利用任一項(非\(a_k\)即可).
例 若等比數列\(\{a_n\}\)中,\(a_4=4\),\(q=2\),則\(a_6=\) .
\(\quad\) 解 \(a_6=a_4\cdot q^2=16\).
(2) \(q^{n-m}=\dfrac{a_n}{a_m}\);
證明 由性質\(a_n=a_m q^{n-m}\)
意義 利用等比數列任意兩項可求公比.
例 若等比數列\(\{a_n \}\)中,\(a_4=4\),\(a_6=16\),則公比\(q=\) .
\(\quad\) 解 \(q^{6-4}=\dfrac{a_6}{a_4}=\dfrac{16}{4}=4 \Rightarrow q^2=4 \Rightarrow q=\pm 2\).
(3) 若\(m+n=p+t\) , 則\(a_m a_n=a_p a_t\);
證明 由等比數列通項公式可得
\(\quad\)\(a_m a_n=a_1 q^{m-1} \cdot a_1 q^{n-1}=a_1^2 q^{n+m-2}\)
\(\quad\)\(a_s a_t=a_1 q^{s-1} \cdot a_1 q^{t-1}=a_1^2 q^{s+t-2}\),
\(\quad\)\(\because m+n=s+t\), \(\therefore a_1^2 q^{n+m-2}=a_1^2 q^{s+t-2}\),
\(\quad\)即\(a_m a_n=a_s a_t\).
意義 下標和相等,其對應項的和相等.
例 \(a_2 a_8=a_1 a_9=a_5^2\),但\(a_2 a_8\)不一定等於\(a_{10}\).
(4) 數列\(\{λa_n\}\)\((λ\)是不為零的常數\()\)仍是公比為\(q\)的等比數列;若數列\(\{b_n\}\)是公比為t的等比數列,則數列\(\{a_n b_n\}\)是公比為\(q\cdot t\)的等比數列;
證明 \(\dfrac{a_{n+1} b_{n+1}}{a_n b_n}=\dfrac{a_{n+1}}{a_n} \cdot \dfrac{b_{n+1}}{b_n}=q \cdot t\),得證.
(5)下標成等比數列且公比為\(m\)的項\(a_k\),\(a_{k+m}\), \(a_{k+2 m}\),\(…\)\((k ,m∈N^*)\)組成公比為\(q^m\)的等比數列.
證明 \(\dfrac{a_{k+(n+1) m}}{a_{k+n m}}=\dfrac{a_1 q^{k+(n+1) m-1}}{a_1 q^{k+n m-1}}=q^m\),得證.
基本方法
【題型1】 等比數列的性質的應用
【典題1】 已知正項等比數列\(\{a_n \}\)中,\(a_2=2\),\(a_5=4a_3\),則\(a_6=\)( )
A.\(16\) \(\qquad \qquad \qquad \qquad\) B.\(32\) \(\qquad \qquad \qquad \qquad\) C. \(64\) \(\qquad \qquad \qquad \qquad\) D.\(-32\)
解析 因為正項等比數列\(\{a_n \}\)中,\(a_2=2\),\(a_5=4a_3\),
所以 \(q^2=\dfrac{a_5}{a_3}=4\),所以\(q=2\),
則\(a_6=a_2⋅q^4=2×2^5=32\).
故選:\(B\).
點撥 本題利用性質\(a_n=a_m q^{n-m}\),沒使用通項公式\(a_n=a_1 q^{n-1}\).
【典題2】 等比數列\(\{a_n \}\)是遞增數列,\(a_2⋅a_4=6\),\(a_1+a_5=5\),則\(a_9\)的值為( )
A. \(\dfrac{9}{2}\) \(\qquad \qquad \qquad \qquad\) B. \(\dfrac{4}{3}\) \(\qquad \qquad \qquad \qquad\) C. \(\dfrac{9}{2}\)或 \(\dfrac{4}{3}\) \(\qquad \qquad \qquad \qquad\) D. \(\dfrac{2}{9}\)或 \(\dfrac{3}{4}\)
解析 \(\because\)在等比數列\(\{a_n \}\)中,\(a_2⋅a_4=6\),\(a_1+a_5=5\),
\(\therefore\left\{\begin{array}{l}
a_2 \cdot a_4=a_1 \cdot a_5=6 \\
a_1+a_5=5
\end{array}\right.\),
即\(a_1\),\(a_5\)是方程\(x^2-5x+6=0\)的兩實數根,
解得 \(\left\{\begin{array}{l}
a_1=2 \\
a_5=3
\end{array}\right.\)或 \(\left\{\begin{array}{l}
a_1=3 \\
a_5=2
\end{array}\right.\),
\(∵\{a_n \}\)是遞增數列, \(\therefore\left\{\begin{array}{l}
a_1=2 \\
a_5=3
\end{array}\right.\), \(\therefore\left\{\begin{array}{l}
a_1=2 \\
q^4=\dfrac{3}{2}
\end{array}\right.\),
\(\therefore a_9=a_1 q^8=2 \times\left(\dfrac{3}{2}\right)^2=\dfrac{9}{2}\).
故選:\(A\).
點撥 本題利用性質: 若\(m+n=p+t\), 則 \(a_m a_n=a_p a_t\);沒使用通項公式\(a_n=a_1 q^{n-1}\).
【典題3】 已知正項等比數列\(\{a_n \}\)滿足:\(a_2 a_8=16a_5\),\(a_3+a_5=20\),若存在兩項\(a_m\),\(a_n\)使得 \(\sqrt{a_m a_n}=32\),則\(\dfrac{1}{m}+\dfrac{4}{n}\)的最小值為\(\underline{\quad \quad}\) .
解析 \(\because a_2 a_8=16a_5\), \(\therefore a_5^2=16a_5\), \(\therefore a_5=16\),
\(∵a_3+a_5=20\),\(\therefore a_3=4\), \(\therefore q^2=\dfrac{a_5}{a_3}=4\),
\(\because\) 正項等比數列\(\{a_n \}\), \(\therefore q=2\),
\(\therefore a_1=1\),\(\therefore a_n=2^{n-1}\),
\(\because \sqrt{a_m a_n}=32\), \(\therefore a_m a_n=2^{10}\), \(\therefore 2^{m+n-2}=2^{10}\),
\(\therefore m+n=12\),
\(\therefore \dfrac{1}{m}+\dfrac{4}{n}=\dfrac{1}{12}\left(\dfrac{1}{m}+\dfrac{4}{n}\right)(m+n)=\dfrac{1}{12}\left(5+\dfrac{n}{m}+\dfrac{4 m}{n}\right)\)
\(\geq \dfrac{1}{12}\left(5+2 \sqrt{\dfrac{n}{m} \cdot \dfrac{4 m}{n}}\right)=\dfrac{3}{4}\),
當且僅當\(\dfrac{n}{m}=\dfrac{4 m}{n}\),即\(m=4\),\(n=8\)時取等號.
【鞏固練習】
1.若等比數列\(\{a_n \}\)各項都是正數,\(a_1=3\),\(a_1+a_2+a_3=21\),則\(a_4+a_5+a_6\)的值為( )
A.\(42\) \(\qquad \qquad \qquad \qquad\) B.\(63\) \(\qquad \qquad \qquad \qquad\) C.\(84\) \(\qquad \qquad \qquad \qquad\) D.\(168\)
2.已知正項等比數列\(\{a_n \}\)滿足\(a_7=a_6+2a_5\),若\(a_m⋅a_n=16a_1^2\),則\(m+n\)的值為( )
A.\(2\) \(\qquad \qquad \qquad \qquad\) B.\(6\) \(\qquad \qquad \qquad \qquad\) C. \(4\) \(\qquad \qquad \qquad \qquad\) D.\(5\)
3.各項均為正數的等比數列\(\{a_n \}\)滿足\(\log_2a_1+\log_2a_2+⋯+\log_2a_{10}=10\),則\(a_5 a_6=\)( )
A.\(8\) \(\qquad \qquad \qquad \qquad\) B.\(4\) \(\qquad \qquad \qquad \qquad\) C. \(-4\) \(\qquad \qquad \qquad \qquad\) D.\(-8\)
4.等比數列\(\{a_n \}\)滿足\(a_8+a_{10}=\dfrac{1}{2}\),\(a_{11}+a_{13}=1\),則 \(a_{20}+a_{22}=\)( )
A.\(8\) \(\qquad \qquad \qquad \qquad\) B.\(4\) \(\qquad \qquad \qquad \qquad\) C. \(-4\) \(\qquad \qquad \qquad \qquad\) D.\(-8\)
5.在等比數列\(\{a_n \}\)中,\(a_3\),\(a_{15}\)是方程\(x^2+6x+2=0\)的根,則 \(\dfrac{a_1 a_{17}}{a_9}\)的值為\(\underline{\quad \quad}\) .
6.在等比數列\(\{a_n \}\)中,\(a_1+a_3=1\),\(a_4+a_6=-8\),則 \(\dfrac{a_{10}+a_{12}}{a_5+a_7}=\) \(\underline{\quad \quad}\).
7.在正項等比數列\(\{a_n \}\)中,\(a_2^2+a_4^2=900-2a_1 a_5\),\(a_6=9a_4\),則 \(a_{2020}\)的個位數字是\(\underline{\quad \quad}\).
參考答案
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答案 \(D\)
解析 根據題意,等比數列\(\{a_n \}\)各項都是正數,\(a_1=3\),\(a_1+a_2+a_3=21\)
則有\(\dfrac{a_1+a_2+a_3}{a_1}=1+q+q^2=7\),求得\(q=2\)或\(-3\)(舍負)
\(\therefore a_4+a_5+a_6=(a_1+a_2+a_3)q^3=21×8=168\);
故選:\(D\). -
答案 \(B\)
解析 設正項等比數列\(\{a_n \}\)的公比為\(q\),易知\(q≠1\),
\(\because a_7=a_6+2a_5\),\(\therefore a_6 q=a_6+2 \dfrac{a_6}{q}\),解得\(q=-1\)或\(2\),
\(\because\) 等比數列\(\{a_n \}\)為正項數列,\(\therefore q>0\),\(\therefore q=2\),
\(\because a_m⋅a_n=16a_1^2\), \(\therefore a_1 2^{m-1} a_1 2^{n-1}=16a_1^2\),
\(\therefore m+n=6\).
故選:\(B\). -
答案 \(B\)
解析 由\(\log_2a_1+\log_2a_2+⋯+\log_2a_{10}=\log_2(a_1 a_2⋯a_{10} )=10\),
得\(a_1 a_2⋯a_{10}=2^{10}\),
又\(\{a_n \}\)是等比數列,得\(a_1 a_{10}=⋯=a_5 a_6\),
所以\((a_5 a_6 )^5=2^{10}\),解得\(a_5 a_6=4\).
故選:\(B\). -
答案 \(A\)
解析 設等比數列\(\{a_n \}\)的公比為\(q\),
由 \(a_{11}+a_{13}=\left(a_8+a_{10}\right) q^3\),得 \(1=\dfrac{1}{2} q^3\),則\(q^3=2\),
所以 \(a_{20}+a_{22}=\left(a_{11}+a_{13}\right) \cdot q^9=1 \times 2^3=8\).
故選:\(A\). -
答案 \(-\sqrt{2}\)
解析 在等比數列\(\{a_n \}\)中,\(a_3\),\(a_{15}\)是方程\(x^2+6x+2=0\)的根,
\(\therefore a_3 a_{15}=2\), \(a_3+a_{15}=-6\),
\(\therefore \dfrac{a_1 a_{17}}{a_9}=\dfrac{a_3 a_{15}}{-\sqrt{a_3 a_{15}}}=-\sqrt{2}\). -
答案 \(-32\)
解析 因為\(\{a_n \}\)為等比數列,\(a_1+a_3=1\),\(a_4+a_6=-8\),
所以\(a_4+a_6=a_1 q^3+a_3 q^3=q^3 (a_1+a_3 )=q^3=-8\),
所以\(q=-2\),
所以 \(\dfrac{a_{10}+a_{12}}{a_5+a_7}=\dfrac{a_5 q^5+a_7 q^5}{a_5+a_7}=q^5=-32\). -
答案 \(7\)
解析 \(\because\)正項等比數列\(\{a_n\}\)中,\(\because a_2^2+a_4^2=900-2a_1 a_5=900-2a_2 a_4\),
\(\therefore (a _2+a _4) ^2=900\),故\(a_2+a_4=30\).
因為\(a_6=9a_4\),故有\(a_4 q^2=9a_4\),
\(\therefore q=3\).
再根據\(a_2 (1+q^2)=30\),求得,\(a_2=3\).
故\(a_{2020}=a_2 q^{2018}=3^{2019}=3^{504 \times 4+3}\),
故\(a_{2020}\)的尾數即為\(3^3\)的尾數,而\(3^3=27\),
故\(a_{2020}\)的個位數為\(7\).
【題型2】 等比數列的綜合問題
【典題1】 有四個實數,前三個數依次成等比數列,它們的積是\(-8\),後三個數依次成等差數列,它們的積為\(-80\),求出這四個數.
解析 由題意設此四個數為 \(\dfrac{b}{q}\),\(b\),\(bq\),\(a\),
則有\(\left\{\begin{array}{l}
b^3=-8 \\
2 b q=a+b \\
a b^2 q=-80
\end{array}\right.\) ,解得 \(\left\{\begin{array}{l}
a=10 \\
b=-2 \\
q=-2
\end{array}\right.\)或 \(\left\{\begin{array}{l}
a=-8 \\
b=-2 \\
q=\dfrac{5}{2}
\end{array}\right.\)
所以這四個數為\(1\),\(-2\),\(4\),\(10\)或 \(-\dfrac{4}{5}\),\(-2\),\(-5\),\(-8\).
【典題2】 在等比數列\(\{a_n \}\)中,公比 \(q∈(0,1)\),且滿足\(a_3=2\),\(a_1 a_3+2a_2 a_4+a_3 a_5=25\).
(1)求數列\(\{a_n \}\)的通項公式;
(2)設\(b_n=\log_2a_n\),數列\(\{b_n \}\)的前\(n\)項和為\(S_n\),當\(\dfrac{S_1}{1}+\dfrac{S_2}{2}+\cdots+\dfrac{S_n}{n}\)取最大值時,求\(n\)的值.
解析 (1)\(\because a_1 a_3+2a_2 a_4+a_3 a_5=25\),\(\therefore a_2^2+2a_2 a_4+a_4^2=25\),
所以\((a_2+a_4 )^2=25\),
\(\because a_3=2\),\(q∈(0,1)\),則對任意的 \(n∈N^*\),可得出 \(a_n>0\),\(\therefore a_2+a_4=5\),
由題意可得\(\left\{\begin{array}{l}
a_3=a_1 q^2=2 \\
a_2+a_4=a_1 q(1+q)=5 \\
q=\dfrac{1}{2}
\end{array}\right.\), \(\left\{\begin{array}{c}
a_1=8 \\
0<q<1
\end{array}\right.\),
因此\(a_n=a_1 q^{n-1}=8 \times\left(\dfrac{1}{2}\right)^{n-1}=2^{4-n}\);
(2) \(b_n=\log_2a_n=\log_22^{4-n}=4-n\),則\(b_{n+1}-b_n=[4-{n+1}]-(4-n)=-1\),
數列\(\{b_n \}\)為等差數列,可得 \(S_n=\dfrac{n\left(b_1+b_n\right)}{2}=\dfrac{n(3+4-n)}{2}=\dfrac{7 n-n^2}{2}\),
\(\therefore \dfrac{S_n}{n}=\dfrac{\dfrac{7 n-n^2}{2}}{n}=\dfrac{7-n}{2}\), 則 \(\dfrac{S_{n+1}}{n+1}-\dfrac{S_n}{n}=\dfrac{7-(n+1)}{2}-\dfrac{7-n}{2}=-\dfrac{1}{2}\),
所以數列\(\left\{\dfrac{S_n}{n}\right\}\)為等差數列,
則 \(\dfrac{S_1}{1}+\dfrac{S_2}{2}+\cdots+\dfrac{S_n}{n}=\dfrac{n\left(\dfrac{S_1}{1}+\dfrac{S_n}{n}\right)}{2}=\dfrac{n\left(3+\dfrac{7-n}{2}\right)}{2}\)\(=\dfrac{13 n-n^2}{4}=-\dfrac{1}{4}\left(n-\dfrac{13}{2}\right)^2+\dfrac{169}{16}\),
由\(n∈N^*\),可得\(n=6\)或\(7\)時, \(\dfrac{S_1}{1}+\dfrac{S_2}{2}+\cdots+\dfrac{S_n}{n}\)取得最大值.
【鞏固練習】
1.已知四個數,前三個數成等差數列,後三個數成等比數列,中間兩數之積為\(16\),前後兩數之積為$-12$8,求這四個數.
2.已知等比數列\(\{a_n \}\)是遞增數列,滿足\(a_4=32\),\(a_3+a_5=80\).
(1)求\(\{a_n \}\)的通項公式;
(2)設\(b_n=\log_2a_n\),若\(b_n\)為數列\(\{c_n \}\)的前\(n\)項積,證明 \(\dfrac{1}{b_n}+\dfrac{1}{c_n}=1\).
3.已知等比數列\(\{a_n \}\)的各項均為正數,且\(a_6=2\),\(a_4+a_5=12\).
(1)求數列\(\{a_n \}\)的通項公式;
(2)設\(b_n=a_1 a_3 a_5…a_{2n-1}\),\(n∈N^*\),求數列\(\{b_n\}\)的最大項.
參考答案
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答案 \(-4,2,8,32\)或\(4,-2,-8,-32\)
解析 設所求四個數為 \(\dfrac{2 a}{q}-a q\), \(\dfrac{a}{q}\),\(aq\), \(a q^3\),
則由已知,得 \(\left\{\begin{array}{l} \left(\dfrac{a}{q}\right)(a q)=16 \\ \left(\dfrac{2 a}{q}-a q\right)\left(a q^3\right)=-128 \end{array}\right.\),
解得\(a=4\),\(q=2\)或\(a=4\),\(q=-2\)或\(a=-4\),\(q=2\)或\(a=-4\),\(q=-2\).
因此所求的四個數為\(-4,2,8,32\)或\(4,-2,-8,-32\). -
答案 (1)\(a_n=2^{n+1}\);(2)略.
解析 (1)設等比數列\(\{a_n \}\)的公比為\(q(q>1)\),
由\(a_4=32\)得 \(a_3+a_5=\dfrac{32}{q}+32 q=80\),解得\(q=2\)或\(q=\dfrac{1}{2}\)(捨去).
所以\(a_n=a_4⋅2^{n-4}=2^{n+1}\).
(2)證明:由\(b_n=\log_2a_n=n+1\),得\(c_1=b_1=2\),
當\(n≥2\)時,\(c_1⋅c_2⋯⋯c_n=n+1\)①,\(c_1⋅c_2⋯⋯c_{n-1}=n\)②,
由①②得\(c_n=\dfrac{n+1}{n}(n \geq 2)\),
當\(n=1\)時, \(c_1=\dfrac{2}{1}=2\)滿足上式,故 \(c_n=\dfrac{n+1}{n}\),
\(\therefore \dfrac{1}{b_n}+\dfrac{1}{c_n}=\dfrac{1}{n+1}+\dfrac{n}{n+1}=1\). -
答案 (1)\(a_n=2^{7-n}\); (2)\(2^{12}\).
解析 (1)設等比數列\(\{a_n \}\)的公比為\(q(q>0)\),
由\(a_6=2\),\(a_4+a_5=12\),
可得 \(\dfrac{a_6}{q^2}+\dfrac{a_6}{q}=12\),即 \(\dfrac{2}{q^2}+\dfrac{2}{q}=12\),解得\(q=\dfrac{1}{2}\)或 \(q=\dfrac{1}{3}\)(捨去),
\(\therefore a_1=\dfrac{a_6}{q^5}=64\),
\(\therefore a_n=64 \times\left(\dfrac{1}{2}\right)^{n-1}=2^{7-n}\);
(2) \(b_n=a_1 a_3 a_5 \ldots a_{2 n-1}=2^6 \times 2^4 \times 2^2 \times \cdots \times 2^{8-2 n}\)
\(=2^{\frac{n(6+8-2 n)}{2}}=2^{-n^2+7 n}=2^{-(n-3.5)^2+12.25}\),
\(\therefore\)當\(n\)取\(3\)或\(4\)時,\(b_n\)取得最大項 \(2^{12}\).
分層練習
【A組---基礎題】
1.已知數列\(\{a_n \}\)滿足\(a_{n+1}=2a_n\),若\(a_4+a_5=3\),則\(a_2+a_3=\)( )
A.\(6\) \(\qquad \qquad \qquad \qquad\) B. \(12\) \(\qquad \qquad \qquad \qquad\) C. \(\dfrac{3}{2}\) \(\qquad \qquad \qquad \qquad\) D. \(\dfrac{3}{4}\)
2.已知等比數列\(\{a_n \}\)中,\(a_2=3\),\(a_6=27\),則\(a_4=\)( )
A. \(-9\) \(\qquad \qquad \qquad \qquad\) B.\(9\) \(\qquad \qquad \qquad \qquad\) C. \(±9\) \(\qquad \qquad \qquad \qquad\) D.\(15\)
3.在等比數列\(\{a_n \}\)中,\(a_2⋅a_5⋅a_8=-27\),則\(a_3⋅a_7=\)( )
A.\(-9\) \(\qquad \qquad \qquad \qquad\) B.\(9\) \(\qquad \qquad \qquad \qquad\) C. \(-27\) \(\qquad \qquad \qquad \qquad\) D.\(27\)
4.正項等比數列\(\{a_n \}\)滿足\(a_2^2+2a_3 a_7+a_6 a_{10}=16\),則\(a_2+a_8=\)( )
A.\(-4\) \(\qquad \qquad \qquad \qquad\) B.\(4\) \(\qquad \qquad \qquad \qquad\) C.\(±4\) \(\qquad \qquad \qquad \qquad\) D.\(8\)
5.(多選)等比數列\(\{a_n \}\)的公比為\(q\),且滿足\(a_1>1\),\(a_{1010} a_{1011}>1\),\(\left(a_{1010}-1\right)\left(a_{1011}-1\right)<0\).記\(T_n=a_1 a_2 a_3…a_n\),則下列結論正確的是( )
A.\(0<q<1\) \(\qquad \qquad \qquad \qquad\) B. \(a_{1010} a_{1012}-1>0\)
C.\(T_n≤T_{1011}\) \(\qquad \qquad \qquad \qquad\) D.使\(T_n<1\)成立的最小自然數\(n\)等於\(2021\)
6.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 \(\underline{\quad \quad}\).
7.已知等比數列\(\{a_n \}\)滿足\(a_2=3\),\(a_2+a_4+a_6=21\),則\(a_4+a_6+a_8=\)\(\underline{\quad \quad}\).
8.已知公比大於\(1\)的等比數列\(\{a_n \}\)中,\(a_2 a_8=8\),\(a_4+a_6=6\),則 \(\dfrac{a_{2020}}{a_{2022}}=\)\(\underline{\quad \quad}\) .
9.已知等比數列\(\{a_n \}\)中\(a_4=1\),若 \(\dfrac{1}{a_1}+\dfrac{1}{a_3}+\dfrac{1}{a_5}+\dfrac{1}{a_7}=6\),則\(a_1+a_3+a_5+a_7=\)\(\underline{\quad \quad}\).
10.等比數列\(\{a_n \}\)中\(a_2+a_7=66\),\(a_3 a_6=128\),求等比數列的通項公式\(a_n\).
11.已知等比數列的項數\(n\)為奇數,且所有奇數項的乘積為\(1024\),所有偶數項的乘積為\(128 \sqrt{2}\),求項數\(n\)的值.
12.在等比數列\(\{a_n \}\)中\(a_n>0(n∈N^* )\),公比\(q∈(0,1)\),且\(a_1 a_5+2a_3 a_5+a_2 a_8=25\),又\(a_3\)與\(a_5\)的等比中項為\(2\).
(1)求數列\(\{a_n \}\)的通項公式;
(2)設\(b_n=\log_2a_n\),數列\(\{b_n\}\)的前\(n\)項和為\(S_n\),求數列\(\{S_n \}\)的通項公式;
(3)當\(\dfrac{S_1}{1}+\dfrac{S_2}{2}+\cdots+\dfrac{S_n}{n}\)取得最大值時,求 \(n\)的值.
參考答案
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答案 \(D\)
解析 因為數列\(\{a_n \}\)滿足\(a_{n+1}=2a_n\),
所以數列\(\{a_n \}\)是公比為\(2\)的等比數列,
因為\(a_4+a_5=3\),
所以\(a_2 q^2+a_3 q^2=(a_2+a_3 ) q^2=4(a_2+a_3 )=3\),
所以 \(a_2+a_3=\dfrac{3}{4}\).
故選:\(D\). -
答案 \(B\)
解析 設等比數列\(\{a_n \}\)的公比為\(q\),
則\(a_2 q^4=a_6\),即 \(q^4=\dfrac{a_6}{a_2}=\dfrac{27}{3}=9\),故\(q^2=3\),
所以\(a_4=a_2 q^2=3×3=9\).
故選:\(B\). -
答案 \(B\)
解析 在等比數列\(\{a_n \}\)中,\(a_2⋅a_5⋅a_8=-27\),
則\(a_5^3=-27\),解得\(a_5=-3\),
所以\(a_3⋅a_7=a_5^2=(-3)^2=9\).
故選:\(B\). -
答案 \(B\)
解析 根據題意,等比數列\(\{a_n \}\)滿足\(a_2^2+2a_3 a_7+a_6 a_{10}=16\),
則有\(a_2^2+2a_2 a_8+a_8^2=16\),即\((a_2+a_8)^2=16\),
又由數列\(\{a_n \}\)為正項等比數列;
故\(a_2+a_8=4\);
故選:\(B\). -
答案 \(AD\)
解析 由 \(\left(a_{1010}-1\right)\left(a_{1011}-1\right)<0\).得 \(\left\{\begin{array}{l} a_{1010}>1 \\ a_{1011}<1 \end{array}\right.\)或 \(\left\{\begin{array}{l} a_{1010}<1 \\ a_{1011}>1 \end{array}\right.\),
又\(a_1>1\);\(a_{1010} a_{1011}>1\),則 \(a_{1010}>1\), \(0<a_{1011}<1\),
所以\(0<q=\dfrac{a_{1011}}{a_{1010}}<1\),\(A\)正確.
由$a_{1010} a_{1012}=a_{1011}^2 $,又 \(0<a_{1011}<1\),
所以\(a_{1010} a_{1012}-1=a_{1011}^2-1<0\),\(B\)錯誤.
由\(a_1>1\), \(a_{1010}>1\), \(0<a_{1011}<1\),
可知當\(n≤1010\)時,\(a_n>1\);當\(n≥1011\)時,\(0<a_n<1\),
所以\(T_n\)的最大值\(T_{1010}\),\(C\)錯誤.
因為 \(T_{2020}=a_1 \times a_2 \times \ldots \times a_{2020}=\left(a_{1010} a_{1011}\right)^{1010}>1\);
\(T_{2021}=a_1 \times a_2 \times \ldots \times a_{2021}=\left(a_{1011}\right)^{2021}<1\),
所以使\(T_n<1\)成立的最小自然數\(n\)等於\(2021\),\(D\)正確.
故選:\(AD\). -
答案 \(192\)
解析 設第\(2\)層浮雕數為\(a_2\),第\(5\)層浮雕數為\(a_5\),
則\(a_2=6\),\(a_5=48\),
\(\because\)各層浮雕數成等比數列,設公比為\(q\),
\(\therefore 48=6×q^3\),\(\therefore q=2\),
\(\therefore\) 第\(7\)層浮雕數為\(a_7=a_5⋅q^2=48×4=192\). -
答案 \(42\)
解析 設等比數列\(\{a_n \}\)的公比為\(q\),\(\because a_2=3\),\(a_2+a_4+a_6=21\),
\(\therefore a_1 q=3\),\(a_1 (q+q^3+q^5)=21\),解得\(q^2=2\).
則\(a_4+a_6+a_8=q^2 (a_2+a_4+a_6)=42\),
故答案為:\(42\). -
答案 \(\dfrac{1}{2}\)
解析 \(\because\)公比大於\(1\)的等比數列\(\{a_n \}\)中,\(a_2 a_8=8\),\(a_4+a_6=6\), \(\therefore a_2 a_8=a_4 a_6=8\),
則\(a_4\) 、\(a_6\)是方程\(t^2-6t+8=0\)的根,
\(\therefore a_4=2\),\(a_6=4\),公比\(q^2=2\), \(\therefore \dfrac{a_{2020}}{a_{2022}}=\dfrac{1}{q^2}=\dfrac{1}{2}\). -
答案 \(6\)
解析 等比數列\(\{a_n \}\)中\(a_4=1\),若 \(\dfrac{1}{a_1}+\dfrac{1}{a_3}+\dfrac{1}{a_5}+\dfrac{1}{a_7}=6\),
則\(\dfrac{a_1+a_7}{a_1 a_7}+\dfrac{a_3+a_5}{a_3 a_5}=\dfrac{a_1+a_3+a_5+a_7}{a_4^2}=6\),\(a_1+a_3+a_5+a_7=6a_4^2=6\).
故答案為:\(6\). -
答案 \(a_n=2^{n-1}\)或\(a_n=2^{8-n}\)
解析 設等比數列的首項為\(a_1\),公比為\(d\),
由題意得 \(\left\{\begin{array} { l } { a _ { 2 } + a _ { 7 } = 6 6 } \\ { a _ { 3 } a _ { 6 } = 1 2 8 } \end{array} \Rightarrow \left\{\begin{array} { l } { a _ { 2 } + a _ { 7 } = 6 6 } \\ { a _ { 2 } a _ { 7 } = 1 2 8 } \end{array} \Rightarrow \left\{\begin{array}{l} a_2=2 \\ a_7=64 \end{array}\right.\right.\right.\)或 \(\left\{\begin{array}{l} a_2=64 \\ a_7=2 \end{array}\right.\),
\(\therefore q^5=\dfrac{a_7}{a_2}=2^5\)或 \(\dfrac{1}{2^5}\),\(\therefore q=2\)或\(\dfrac{1}{2}\).
\(\therefore a_n=a_2 q^{n-2}=2^{n-1}\)或 \(\dfrac{1}{2^{n-8}}\).
\(\therefore a_n=2^{n-1}\)或 \(a_n=2^{8-n}\). -
答案 \(7\)
解析 根據題意,設該等比數列為\(\{a_n \}\),\(n\)為奇數;
若所有奇數項的乘積為\(1024\),則有\(a_1 a_3 a_5…a_n=1024\),①,
所有偶數項的乘積為 \(128 \sqrt{2}\),則有 \(a_2 a_4 a_6 \ldots a_{n-1}=128 \sqrt{2}\),②
①÷②可得: \(a_1 q^{\dfrac{n-1}{2}}=\dfrac{1024}{128 \sqrt{2}}=4 \sqrt{2}\),即 \(\dfrac{a_{n+1}}{2}=4 \sqrt{2}\) ③
①×②可得: \(\left(a_1 a_n\right) \cdot\left(a_2 a_{n-1}\right) \cdots \dfrac{a_{n+1}^2}{}=1024 \times 128 \sqrt{2}\),
即\(\left(\dfrac{a_{n+1}}{2}\right)^n=1024 \times 128 \sqrt{2}\) ④
聯立③④可得: \((4 \sqrt{2})^n=1024 \times 128 \sqrt{2}\),
解可得:\(n=7\). -
答案 (1) \(a_n=2^{5-n}\);(2) \(S_n=\dfrac{n(9-n)}{2}\);(3)\(n=8\)或\(n=9\).
解析 (1) \(\because a_1 a_5+2a_3 a_5+a_2 a_8=25\), \(\therefore a_3^2+2a_3 a_5+a_5^2=25\)
又 \(\because a_n>0\),\(\therefore a_3+a_5=5\) (1)
\(\because a_3\)與 \(a_5\) 的等比中項為 \(2\) , \(\therefore a_3 a_5=4\),
\(\because q∈(0,1)\),\(\therefore a_3>a_5\),
\(\therefore\) 由(1)(2)解得\(a_3=4\),\(a_5=1\),
\(\therefore q^2=\dfrac{a_5}{a_3}=\dfrac{1}{4}\),解得 \(q=\dfrac{1}{2}\),
\(\therefore a_1=16\), \(\therefore a_n=16 \times\left(\dfrac{1}{2}\right)^{n-1}=2^{5-n}\),
(2)\(\because b_n=\log_2 a_n =5-n\),\(\therefore b_{n+1}-b_n=-1\),\(b_1=4\),
\(\therefore\) 數列 \(\{b_n \}\)是以 \(b_1=4\)為首項, \(-1\)為公差的等差數列.
\(\therefore S_n=\dfrac{n(9-n)}{2}\),
(3) 由 \(S_n=\dfrac{n(9-n)}{2}\),得\(\dfrac{S_n}{n}=\dfrac{9-n}{2}\),
當\(n≤8\)時, \(\dfrac{S_n}{n}>0\);當 \(n=9\)時,\(\dfrac{S_n}{n}=0\);當 \(n>9\)時, \(\dfrac{S_n}{n}<0\)
\(\therefore\)當 \(\dfrac{S_1}{1}+\dfrac{S_2}{2}+\cdots+\dfrac{S_n}{n}\)取得最大值時, \(n=8\)或\(n=9\).
【B組---提高題】
1.(多選)已知等比數列\(\{a_n \}\)的各項均為正數,公比為\(q\),且\(a_1>1\),\(a_6+a_7>a_5 a_8+1>2\),記\(\{a_n \}\)的前\(n\)項積為\(T_n\),則下列選項中正確的選項是( )
A.\(0<q<1\) \(\qquad \qquad \qquad\) B.\(a_6>1\) \(\qquad \qquad \qquad\) C.\(T_{12}>1\) \(\qquad \qquad \qquad\) D.\(T_{13}>1\)
2.在等比數列\(\{a_n \}\)中,\(a_1<0\),\(a_2 a_4+2a_3 a_5+a_4 a_6=25\),求\(a_3+a_5=\)\(\underline{\quad \quad}\).
參考答案
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答案 \(ABC\)
解析 \(\because a_6+a_7>a_5 a_8+1\), \(\therefore a_6+a_7>a_6 a_7+1\),
\(\therefore (a_6-1)(a_7-1)<0 \quad (*)\),
\(\because a_1>1\),若\(a_6<1\),則一定有\(a_7<1\),不符合\((*)\),
則\(a_6>1\),\(a_7<1\), \(\therefore 0<q<1\).
\(\because a_6 a_7+1>2\), \(\therefore a_6 a_7>1\),
\(T_{12}=a_1 a_2 a_3 \ldots a_{12}=\left(a_6 a_7\right)^6>1\),\(T_{13}=a_7^{13}<1\),
\(\therefore\) 滿足\(T_n>1\)的最大正整數\(n\)的值為\(12\).
故選:\(ABC\). -
答案 -5
解析 \(\because \{a_n\}\)是等比數列,且\(a_1<0\),\(a_2 a_4+2a_3 a_5+a_4 a_6=25\),
\(\therefore a_3^2+2a_3 a_5+a_5^2=25\),即\((a_3+a_5 )^2=25\).
再由\(a_3=a_1 q^2<0\),\(a_5=a_1 q^4<0\),\(q\)為公比,可得\(a_3+a_5=-5\),
故答案為:\(-5\).
【C組---拓展題】
1.設\(a\),\(b∈R\),關於\(x\)的方程\((x^2-ax+1)(x^2-bx+1)=0\)的四個實根構成以\(q\)為公比的等比數列,若\(q \in\left[\dfrac{1}{2}, \sqrt{2}\right]\),則\(ab\)的取值範圍為\(\underline{\quad \quad}\) .
參考答案
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答案 \(\left[4, \dfrac{27}{4}\right]\)
解析 設方程\((x^2-ax+1)(x^2-bx+1)=0\)的\(4\)個實數根依次為\(m\),\(mq\),\(mq^2\),\(mq^3\),
由等比數列性質,不妨設\(m\),\(mq^3\)為\(x^2-ax+1=0\)的兩個實數根,
則\(mq\),\(mq^2\)為方程\(x^2-bx+1=0\)的兩個根,
由韋達定理得,\(m^2 q^3=1\),\(m+mq^3=a\),\(mq+mq^2=b\),則 \(m^2=\dfrac{1}{q^3}\) ,
故\(ab=(m+mq^3)(mq+mq^2)=m^2 (1+q^3)(q+q^2)\),
\(=\dfrac{1}{q^3}\left(1+q^3\right)\left(q+q^2\right)=q+\dfrac{1}{q}+q^2+\dfrac{1}{q^2}\) ,
設 \(t=q+\dfrac{1}{q}\),則 \(q^2+\dfrac{1}{q^2}=t^2-2\),
因為\(q \in\left[\dfrac{1}{2}, \sqrt{2}\right]\),且 \(t=q+\dfrac{1}{q}\)在 \(\left[\dfrac{1}{2}, 1\right]\)上遞減,在 \((1, \sqrt{2}]\)上遞增,
當\(q=\dfrac{1}{2}\)時, \(t=\dfrac{5}{2}\),當 \(t=\sqrt{2}\)時, \(t=\dfrac{3 \sqrt{2}}{2}\)
所以 \(t \in\left[2, \dfrac{5}{2}\right]\),
則 \(a b=t^2+t-2=\left(t+\dfrac{1}{2}\right)^2-\dfrac{9}{4}\),
所以當\(t=2\)時,\(ab\)取到最小值是\(4\),
當\(t=\dfrac{5}{2}\)時,\(ab\)取到最大值是 \(\dfrac{27}{4}\),
所以\(ab\)的取值範圍是\(\left[4, \dfrac{27}{4}\right]\).
故答案為:\(\left[4, \dfrac{27}{4}\right]\).