M: Can I help you, Madam? W: Yes, I want to report a theft that happened to my house during our absence. My husband and I went on a holiday last week. M: I see. What is missing? Who is the woman speaking to?
【单选题】在等差数列{ a n }中,给出以下结论: ①恒有: a 2 + a 8 ≠ a 10 ; ②数列{ a n }的前 n 项和公式不可能是 S n = n ; ③若 m , n , l , k ∈N * ,则“ m + n = l + k ”是“ a m + a n = a l + a k ”成立的充要条件; ④若 a 1 =12, S 6 = S 11 ,则必有 a 9 =0,其中正确的是( ...