已知等差数列{an}的前n项和为Sn，且a3+2a6+a9=15，则S11等于[_爱下问搜题

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问题选择题

已知等差数列{a_n}的前n项和为S_n，且a₃+2a₆+a₉=15，则S₁₁等于[ ]

A．78

B．66

C．55

D．33

答案

答案：D

填空题

(A) 条件(1)充分，但条件(2)不充分．
(B) 条件(2)充分，但条件(1)不充分．
(C) 条件(1)和(2)单独都不充分，但条件(1)和条件(2)联合起来充分．
(D) 条件(1)充分，条件(2)也充分．
(E) 条件(1)和条件(2)单独都不充分，条件(1)和条件(2)联合起来也不充分．

Class	Average Age	No. of Students
A	15 years	6
B	16 years	12

Is the standard deviation of ages of students in class A greater than the standard deviation of the age of students in class B
(1) The difference between the ages of any two students in class A is always more than 1 year.
(2) No Student in class B is more than 6 months older than any other student.

单项选择题

质量改进是质量控制的（）。

A、替代

B、补充

C、基础

D、一部分