n=log3a1+log3a2+...+log3an
=log(3)(a1a2a3a4a5a6..an)
=log(3)(1/3×1/3^2+1/3^3×...×1/3^n)
=log(3)[3^(-1-2-3-..-n)
=-n(n+1)/2
∴1/bn=-2/n(n+1)=-2[1/n-1/(n+1)]
∴Sn=-2×[1-1/2+1/2-1/3+..+1/n-1/(n+1)]
=-2×(1-1/(n+1)
=-2n/(n+1)
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