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an =a1+(n-1)d = a1+(n-1)
(1)
1,a1,a3成等比数列
a3 = (a1)^2
a1+2 = (a1)^2
(a1)^2-a1-2 =0
(a1-2)(a1+1) =0
a1=2 or -1
(2)
S5 > a1a9
(a1+2)5 > a1(a1+8)
(a1)^2 +3a1 -10 <0
(a1+5)(a1-2)<0
-5<a1<2
(1)
1,a1,a3成等比数列
a3 = (a1)^2
a1+2 = (a1)^2
(a1)^2-a1-2 =0
(a1-2)(a1+1) =0
a1=2 or -1
(2)
S5 > a1a9
(a1+2)5 > a1(a1+8)
(a1)^2 +3a1 -10 <0
(a1+5)(a1-2)<0
-5<a1<2
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