练习册

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    10． 等差数列中.且成等比数列.求数列前20项的和． 解:设数列的公差为.则 . . ．····························································································· 3分 由成等比数列得. 即. 整理得. 解得或．····································································································· 7分 当时.．·················································································· 9分 当时.. 于是．······················································ 12分 【查看更多】

     

    题目列表(包括答案和解析)

    已知数列是首项a且公比q不等于1的等比数列，是其前n项和，成等差数列．

    (1)证明成等比数；

    (2)求和：．

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    已知递增等差数列中，且是的等比中项，则它的第4项到第11项的和为

    A、180              B、198          C、189          D、168

     

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    已知递增等差数列中，且是的等比中项，则它的第4项到第11项的和为

    A．180B．198C．189D．168

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    已知递增等差数列中，且是的等比中项，则它的第4项到第11项的和为
    A．180B．198C．189D．168

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    在△ABC中，若a、b、c成等比数例，且c=2a，则cosB等于（　　）

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