数列{an}中，an＋1＋(－1)nan＝2n－1，则数列{an}的前12项和等

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问题选择题

数列{a_n}中，a_n_＋1＋(－1)ⁿa_n＝2n－1，则数列{a_n}的前12项和等于(　　)

A．76

B．78

C．80

D．82

答案

答案：B

由已知a_n_＋1＋(－1)ⁿa_n＝2n－1，得a_n_＋2＋(－1)ⁿ^＋1a_n_＋1＝2n＋1，得a_n_＋2＋a_n＝(－1)ⁿ(2n－1)＋(2n＋1)．取n＝1,5,9及n＝2,6,10，结果相加可得S₁₂＝a₁＋a₂＋a₃＋a₄＋…＋a₁₁＋a₁₂＝78.

画图题

1.	four six seven chairs desks
2.	five six eight apples grapes
3.	Open Close your mouth eye
4.	in on under the desk chair
5.	Open Close the book pencil case

选择题

--- Did you see a girl in yellow pass by just now?

--- No, sir. I ________ a magazine.

A．read

B．am reading

C．would read

D．was reading