数列__高考题数列(高考题) 1. (福建卷)已知等差数列 中, 的值是 ( A ) A.15 B.30 C.31 D.64 2. (湖南卷)已知数列 满足 ,则 = (B ) A.0 B. C. D. 3. (江苏卷)在各项都为正数的等比数列{an}中,首项a1=3 ,前三项和为21,则a3+ a4+ a5=(C ) ( A ) 33 ( B ) 72 ( C ) 84 ( D )189 4. (全国卷II) 如果数列 是等差数列,则(B )...
0且-1< <0或 >0 当 或 时 即 当 且 ≠0时, 即 当 或 =2时, 即 21. (全国卷II) 已知 是各项为不同的正数的等差数列, 、 、 成等差数列.又 , . (Ⅰ) 证明 为等比数列; (Ⅱ) 如果数列 前3项的和等于 ,求数列 的首项 和公差 . (I)证明:∵ 、 、 成等差数列 ∴2 = + ,即 又设等差数列 的公差为 ,则( - ) = ( -3 ) 这样 ,从而 ( - )=0 ∵ ≠0 ∴ = ≠0 ∴ ∴ 是首项为 = ,公比为 的等比数列。 (II)解。∵ ∴ =3 ∴ = =3 _1182916136.unknown _1182916232.unknown _1182916271.unknown _1182916306.unknown _1182916318.unknown _1182916322.unknown _1182916324.unknown _1182916325.unknown _1182916323.unknown _1182916320.unknown _1182916321.unknown _1182916319.unknown _1182916311.unknown _1182916313.unknown _1182916314.unknown _1182916312.unknown _1182916309.unknown _1182916310.unknown _1182916308.unknown _1182916298.unknown _1182916302.unknown _1182916304.unknown _1182916305.unknown _1182916303.unknown _1182916300.unknown _1182916301.unknown _1182916299.unknown _1182916275.unknown _1182916278.unknown _1182916279.unknown _1182916277.unknown _1182916273.unknown _1182916274.unknown _1182916272.unknown _1182916249.unknown _1182916257.unknown _1182916266.unknown _1182916269.unknown _1182916270.unknown _1182916268.unknown _1182916260.unknown _1182916265.unknown _1182916259.unknown 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