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n>=2 S(n-1)=2(n-1)^2-3(n-1)-5 =2n^2-7n an=Sn-S(n-1)=10n-4 a1=S1=2+3-4=1 不符合n>=2時的an=10n-4 所以 an= 1,n=1 10n-4,n≥2
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