It is known that SN is the sum of the first n terms of the equal ratio sequence {an}, S3, S9 and S3 are equal difference sequence, and A2, a8 and A5 are equal difference sequence It's a long story

It is known that SN is the sum of the first n terms of the equal ratio sequence {an}, S3, S9 and S3 are equal difference sequence, and A2, a8 and A5 are equal difference sequence It's a long story

The title should be S3, S9, S6 into arithmetic sequence proof: first, according to the general formula of arithmetic sequence an = a1 + (n-1) d, then a (n + 1) = a1 + (n) d a (n + 2) = a1 + (n + 1) d. obviously, arithmetic sequence has an + a (n + 2) = 2A (n + 1) according to the summation formula of arithmetic sequence: SN = A1 / (1-Q) - A1 / (1-Q) * q ^ n, then s