问题
选择题
等差数列{an}的前n项和为Sn，若
|
答案
∵
=a6 a4 7 11
∴
=2a6 2a4 7 11
即
=a1+a11 a1+a7 7 11
则
=S11 S7
=11(a1+a11) 2 7(a1+a7) 2
=111(a1+a11) 7(a1+a7)
故选B
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等差数列{an}的前n项和为Sn，若
|
∵
=a6 a4 7 11
∴
=2a6 2a4 7 11
即
=a1+a11 a1+a7 7 11
则
=S11 S7
=11(a1+a11) 2 7(a1+a7) 2
=111(a1+a11) 7(a1+a7)
故选B