设Sn是等差数列{an}的前n项和,已知a2=3,a6=11,则S7=?
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设Sn为等差数列{An}的前n项和,求证：{Sn/n}是等差数列
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设Sn是等差数列{an}的前n项和,已知a2=3,a6=11,则S7=?
设Sn是等差数列｛an｝的前n项和,已知a2=3,a6=11,则S7等于?
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设Sn是等差数列｛an｝的前n项和,已知a2=3,S11=121,则S7=
(1)已知数列an的前n项和为sn满足sn=an²+bn,求证an是等差数列(2)已知等差数列an的前n项和为sn,求证数列sn/n也成等差数列
设Sn是等差数列{an}的前n项和,已知S8=108,Sn=630,S(n-8)=234,求n.