Question

已知等差数列{a n }的公差d≠0,且a 1 ,a 3 ,a 9 成等比数列,则 =(　　) A． B． C． D

已知等差数列{a n }的公差d≠0,且a 1 ,a 3 ,a 9 成等比数列,则 =(　　) A． B． C． D．

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Answer 1

C

等差数列{a n }中,a 1 =a 1 ,a 3 =a 1 +2d,a 9 =a 1 +8d,因为a 1 ,a 3 ,a 9 恰好构成某等比数列,所以有 =a 1 a 9 ,即(a 1 +2d) 2 =a 1 (a 1 +8d),解得d=a 1 ,所以该等差数列的通项为a n =nd.则 的值为 .