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将数列\(\left\{ {{a_n}} \right\}\)中的所有项排成如下数阵:其中每一行项数是上一行项数的2倍,且从第二行起每一行均构成公比为2的等比数列.
\({a_1}\)
\({a_2}\),\({a_3}\)
\({a_4}\),\({a_5}\),\({a_6}\),\({a_7}\)
\({a_8}\),\({a_9}\),\({a_{10}}\),\({a_{11}}\),\({a_{12}}\),\({a_{13}}\),\({a_{14}}\),\({a_{15}}\)
\( \cdot \cdot \cdot \cdot \cdot \cdot \)
记数阵中的第1列 \({a_1}\),\({a_2}\),\({a_4}\),\( \cdot \cdot \cdot \) 构成的数列为 \(\left\{ {{b_n}} \right\}\) , \({T_n}\) 为数列 \(\left\{ {{b_n}} \right\}\) 的前 \(n\) 项和,\({T_n} = 5{n^2} + 3n\) ,则 \({b}_{n}=\) ___,\({a}_{1025}=\) ___.
参考答案:\(10n - 2\);216
解析:
\({T_n}\) 为数列 \(\left\{ {{b_n}} \right\}\) 的前\(n\)项的和, \({T_n} = 5{n^2} + 3n\) ,
\({b_n} = {T_n} - {T_{n - 1}} = \left( {5{n^2} + 3n} \right) - \left[ {5{{\left( {n - 1} \right)}^2} + 3\left( {n - 1} \right)} \right] = 10n - 2\left( {n \geqslant 2} \right)\)
验证 \(n = 1\) 时, \({b_1} = {T_1} = 8\) 也符合,故 \({b_n} = 10n - 2\) , \(\because {b_n} = {a_{{2^{n - 1}}}}\) , \(\therefore {a_{1024}} = {b_{11}} = 108\) ,\({a_{1025}} = 2{a_{1024}} = 216\) .
故答案为: \(10n - 2\) ; 216