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20 0 4年高考数学有这么一道考题 (全国卷Ⅲ第 2 2题 ) :已知数列 {an}的前n项和Sn 满足Sn =2an +( -1) n,n≥ 1.( 1)写出数列 {an}的前三项a1 ,a2 ,a3;( 2 )求数列 {an}的通项公式 .( 3 )证明 :对任意的整数m 4,有1a4 +1a5+… +1am 78.显然 ,本题的前两问考查由Sn 求an,第(
The college entrance examination mathematics in 2004 has such an exam question (National Volume III Question 22): Knowing that the first n terms of the series {an} and Sn satisfy Sn = 2an + (-1)n,n≥ 1.( 1) Write the first three terms of the series {an}, a1, a2, a3; (2) Find the general term of the series {an}. (3) Prove that for any integer m 4, there is 1a4 +1a5+... +1am 78. Obviously, the first two questions in this question were examined by Sn.