共50条信息
已知正项等比数列\(\left\{ {{a}_{n}} \right\}\)的前\(n\)项和为\({{S}_{n}}\),且\({{a}_{1}}{{a}_{6}}=2{{a}_{3}}\),\({{a}_{4}}\)与\({{a}_{6}}\)的等差中项为\(5\),则\({{S}_{5}}=\)( )
已知数列\(\{a_{n}\}\)的前\(n\)项和为\(S_{n}\),\(a_{1}=1\),\(a_{2}=2\),且\(a_{n+2}-2a_{n+1}+a_{n}=0(n∈N*)\),记\({{T}_{n}}=\dfrac{1}{{{S}_{1}}}+\dfrac{1}{{{S}_{2}}}+\cdots \dfrac{1}{{{S}_{n}}}\),则\(T_{2018}=(\) \()\)
若实数数列:\(-1\),\(a\),\(b\),\(m\),\(7\)成等差数列,则圆锥曲线\( \dfrac{x^{2}}{a^{2}}- \dfrac{y^{2}}{b^{2}}= 1\)的离心率为\((\) \()\)
已知数列\(\{a_{n}\}\)中,\(a_{1}=1\),\({{a}_{n+1}}=\dfrac{2{{a}_{n}}}{2+a}(n\in {{N}_{+}})\).
\((\)Ⅰ\()\)求\(a_{2}\),\(a_{3}\),\(a_{4}\)的值,猜想数列\(\{a_{n}\}\)的通项公式;
\((\)Ⅱ\()\)运用\((\)Ⅰ\()\)中的猜想,写出用三段论证明数列\(\{\dfrac{1}{{{a}_{n}}}\}\)是等差数列时的大前提、小前提和结论.
\((2)\)设\({{b}_{n}}=\dfrac{3}{{{a}_{n}}{{a}_{n+1}}}\),求数列\(\{{{b}_{n}}\}\)的前\(n\)项和\({{T}_{n}}\).
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