已知
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是首項為19,公差為-2的等差數(shù)列,
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為
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的前
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項和.
(1)求通項
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及
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;
(2)設(shè)
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是首項為1,公比為3的等比數(shù)列,求數(shù)列
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的通項公式及其前
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項和
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.
(1)
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(2)
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.
(1)直接利用
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,
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求解.
(2)由題意可求出
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,結(jié)合(1)中a
n,可求出b
n,然后再采用分組求和求T
n.
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
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
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
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
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
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
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.
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
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
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
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
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
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