若{an}是个等差数列,前n项和为sn,已知S4=6 S8=20 求S12
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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
若{an}是个等差数列,前n项和为sn,已知S4=6 S8=20 求S12
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