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Let the terms a(1),a(2),a(3),…a(n) be in...

Let the terms `a_(1),a_(2),a_(3),…a_(n)` be in G.P. with common ratio r. Let `S_(k)` denote the sum of first k terms of this G.P.. Prove that `S_(m-1)xxS_(m)=(r+1)/r`underset(ileiltjlen(SigmaSigma)a_(i)a_(j)`

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