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试题 已知数列 {an}是首项为a且公比q不等于 1的等比数列 ,Sn 是其前n项和 ,a1、2a7、3a4 成等差数列 .(Ⅰ )证明 1 2S3、S6 、S12 -S6 成等比数列 ;(Ⅱ )求和Tn =a1+ 2a4 + 3a7+… +na3n- 2 .该题源于教材习题 ,难易适中 ,可运用多种方法求解 ,体现了“重基础、出活题、考能?
The test series {an} is a geometric sequence whose first term is a and the public ratio q is not equal to 1. Sn is the first n terms and a1, 2a7, and 3a4 are equal arithmetic series. (I) Proof 1 2S3, S6 , S12 -S6 into a geometric sequence; (II) Summation Tn =a1+ 2a4 + 3a7+... +na3n- 2. This problem is derived from the textbook exercises, is moderately difficult, and can be solved by a variety of methods, reflecting the "re-foundation, A live question, test can?