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等差数列{a¬n}中,S¬30=50,S¬50=30则S¬80等于多少?

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等差数列{a¬n}中,S¬30=50,S¬50=30则S¬80等于多少?
等差数列{a¬n}中,S¬30=50,S¬50=30则S¬80等于多少?
s(n)/n是等差数列
求吧
设bn=sn/n
b30=...
b50=...
b80=...
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