在正项等比数列 $\left\{ {a_n}\right\} $ 中,${a_5} = \dfrac{1}{2}$,${a_6} + {a_7} = 3$,则满足 ${a_1} + {a_2} + \cdots + {a_n} > {a_1}{a_2} \cdots {a_n}$ 的最大正整数 $n$ 的值为 .
【难度】
【出处】
2013年高考江苏卷
【标注】
【答案】
$12$
【解析】
首先,确定出数列 $\{a_n\}$ 的通项公式,求出前 $n$ 项和与前 $n$ 项积,可利用指数函数的单调性进行比较.设 $\left\{ {a_n} \right\}$ 的公比为 $q\left( {q > 0} \right)$,则由已知并根据等比数列通项公式可得\[{\begin{cases}
{a_1}{q^4} = \dfrac{1}{2} ,\\
\dfrac{1}{2}\left( {q + {q^2}} \right) = 3 ,\\
\end{cases}}\]解得\[{\begin{cases}{a_1} = \dfrac{1}{32}, \\
q = 2 ,\\
\end{cases}}\]于是根据等比数列通项及求和公式得\[\begin{split} {a_1} + {a_2} + \cdots + {a_n} &= \dfrac{{\dfrac{1}{32}\left( {1 - {2^n}} \right)}}{1 - 2} = \dfrac{1}{32}\left( {{2^n} - 1} \right) ,\\ {a_1}{a_2} \cdots {a_n} &= a_1^n{q^{\frac{{n\left( {n - 1} \right)}}{2}}} = {\left( {\dfrac{1}{32}} \right)^n}{2^{\frac{{n\left( {n - 1} \right)}}{2}}}.\end{split} \]由 ${a_1} + {a_2} + \cdots + {a_n} > {a_1}{a_2} \cdots {a_n}$ 可得\[\frac{1}{32}\left( {{2^n} - 1} \right) > {\left( {\frac{1}{32}} \right)^n}{2^{\frac{{n\left( {n - 1} \right)}}{2}}},\]整理得\[{2^n} - 1 > {2^{\frac{1}{2}{n^2} - \frac{11}{2}n + 5}}.\]由 ${2^n} > {2^{\frac{1}{2}{n^2} - \frac{11}{2}n + 5}}$ 可得\[n > \frac{1}{2}{n^2} - \frac{11}{2}n + 5,\]即\[{n^2} - 13n + 10 < 0,\]解此不等式得\[\dfrac{{13 - \sqrt {129} }}{2} < n < \dfrac{{13 + \sqrt {129} }}{2},\]取 $n = 12$,可以验证当 $n = 12$ 时满足\[{a_1} + {a_2} + \cdots + {a_n} > {a_1}{a_2} \cdots {a_n},\]$n \geqslant 13$ 时不满足\[{a_1} + {a_2} + \cdots + {a_n} > {a_1}{a_2} \cdots {a_n},\]故 $n$ 的最大值为 $ 12 $.
{a_1}{q^4} = \dfrac{1}{2} ,\\
\dfrac{1}{2}\left( {q + {q^2}} \right) = 3 ,\\
\end{cases}}\]解得\[{\begin{cases}{a_1} = \dfrac{1}{32}, \\
q = 2 ,\\
\end{cases}}\]于是根据等比数列通项及求和公式得\[\begin{split} {a_1} + {a_2} + \cdots + {a_n} &= \dfrac{{\dfrac{1}{32}\left( {1 - {2^n}} \right)}}{1 - 2} = \dfrac{1}{32}\left( {{2^n} - 1} \right) ,\\ {a_1}{a_2} \cdots {a_n} &= a_1^n{q^{\frac{{n\left( {n - 1} \right)}}{2}}} = {\left( {\dfrac{1}{32}} \right)^n}{2^{\frac{{n\left( {n - 1} \right)}}{2}}}.\end{split} \]由 ${a_1} + {a_2} + \cdots + {a_n} > {a_1}{a_2} \cdots {a_n}$ 可得\[\frac{1}{32}\left( {{2^n} - 1} \right) > {\left( {\frac{1}{32}} \right)^n}{2^{\frac{{n\left( {n - 1} \right)}}{2}}},\]整理得\[{2^n} - 1 > {2^{\frac{1}{2}{n^2} - \frac{11}{2}n + 5}}.\]由 ${2^n} > {2^{\frac{1}{2}{n^2} - \frac{11}{2}n + 5}}$ 可得\[n > \frac{1}{2}{n^2} - \frac{11}{2}n + 5,\]即\[{n^2} - 13n + 10 < 0,\]解此不等式得\[\dfrac{{13 - \sqrt {129} }}{2} < n < \dfrac{{13 + \sqrt {129} }}{2},\]取 $n = 12$,可以验证当 $n = 12$ 时满足\[{a_1} + {a_2} + \cdots + {a_n} > {a_1}{a_2} \cdots {a_n},\]$n \geqslant 13$ 时不满足\[{a_1} + {a_2} + \cdots + {a_n} > {a_1}{a_2} \cdots {a_n},\]故 $n$ 的最大值为 $ 12 $.
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