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a1+a2+a3=7 得a1+a1*q+a1*q^2=q^2+q+1=7 所以(q-2)(q+3)=0 得q=2或q=-3(an>0,故舍去) 所以an=a1q^(n-1)=2^(n-1) Sn=a1(1-q^n)/(1-q)=2^n-1
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