问题
选择题
设Sn是等差数列{an}的前n项和，若
|
答案
由题意可得
=S11 S9 11(a1+a11) 2 9(a1+a9) 2
=
=11×2a6 2 9×2a5 2
=11a6 9a5
×11 9
=19 11
故选A
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设Sn是等差数列{an}的前n项和，若
|
由题意可得
=S11 S9 11(a1+a11) 2 9(a1+a9) 2
=
=11×2a6 2 9×2a5 2
=11a6 9a5
×11 9
=19 11
故选A
