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設公比為k (a4+a6)/(a1+a3)=k^3 5/40=k^3 k=1/2 a3=a1*k^2=a1/4,代入a1+a3=10 解得a1=8 囙此a2=4 另外a3=2,a4=1,a5=1/2 S5=a1+a2+…+a5=31/2
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