a(n) = aq^(n-1), n = 1,2,...
27 = a(1)a(5)a(9) = a*aq^4*aq^8 = a^3q^12.
[a(7)]^2/a(9) = [aq^6]^2/[aq^8] = aq^4 = [a^3q^12]^(1/3) = 27^(1/3)
= 3
已知等比数列An满足a1a5a9=27,则(a7)2/a9=?
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