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If the sum of the first n terms of the arithmetic sequence {an} is Sn, and S4 = 20, sn-4 = 60, Sn = 120, then n=______ ．
Since S4 = 20, sn-4 - S4 = 60 - S4 = 40, sn-sn-4 = 60, х S4, sn-4-s4, sn-sn-4 form an arithmetic sequence. Since the sum of every four items in the arithmetic sequence also forms an arithmetic sequence, х n = 12, the answer is: 12
Let the sum of the first n terms of the arithmetic sequence {an} be Sn, and S4 = 4s2
Let the sum of the first n terms of the arithmetic sequence {an} be Sn, and Sn = 4s2, an = 2An + 1
(1) Find the general term formula of sequence {an};
(2) Let the first n terms and TN of the sequence {BN}, and TN + = λ (λ is a constant), let CN = B2, (n ∈ n ·). Find the first n terms and RN of the sequence {CN}
The stem of the question should be S4 = 4s2, the first question an = 2N-1, and the second answer is as follows:
Given the arithmetic sequence an, the sum of the first n terms is Sn, A5 = - 13, S4 = - 82
Seeking S6
Find the minimum value of Sn
(1) Let a 5 = a1 + 4D = - 13s4 = a1 + A2 + a3 + A4 = a1 + A1 + D + A1 + 2D + A1 + 3D = 4A1 + 6D = - 82, then a 1 = - 25 d = 3, so an = - 28 + 3N Sn = (a1 + an) n / 2 = (- 25-28 + 3n) n / 2 = (- 53 + 3n) n / 2s6 = (- 53 + 18) 6 / 2 = - 105 (2), an =
It is known that the sum of the first five terms of an arithmetic sequence is zero. The sum of the first ten terms is negative 100. Find the sum of the first twenty terms of this sequence
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S10-S5=a6+a7+a8+a9+a10=-100
5a8=-100
a8=-20
S5=a1+a2+a3+a4+a5=5a3=0
a3=0
d=(a8-a3)/5=-4
a1=a3-2d=8
S20=20a1+[20*(20-1)/2]*d
=160-4*190
=-600
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