共50条信息
数列\(\{ a_{n}\}\)中,\(a_{1}{=}1{,}a_{n{+}1}{=}2a_{n}{+}2\),则\(a_{7}\)的值为\(({ })\)
已知等差数列\(\left\{ {{a}_{n}} \right\}\)的公差为\(5\),前\(n\)项和为\({{S}_{n}}\),且\({{a}_{1}}\),\({{a}_{2}}\),\({{a}_{5}}\)成等比数列,则\({{S}_{6}}\) \(=(\) \()\)
已知等比数列\(\{ a_{n}\}\)满足\(a_{1}{=}\dfrac{1}{4}{,}a_{3}a_{5}{=}4(a_{4}{-}1)\),则\(a_{2}{=}({ })\)
已知等差数列\(\left\{{a}_{n}\right\} \)的公差和首项都不等于\(0 \),且\({a}_{2} \),\({a}_{4} \),\({a}_{8} \)成等比数列,则\( \dfrac{{a}_{1}+{a}_{5}+{a}_{9}}{{a}_{2}+{a}_{3}} \)等于\((\) \()\)
设\(\{a_{n}\}\)是由正数组成的等比数列,\(S_{n}\)为其前\(n\)项和\(.\)已知\(a_{2}a_{4}=1\),\(S_{3}=7\),则\(S_{5}=(\) \()\)
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