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If Pn is the sum of a GdotPdot upto n te...

If `P_n` is the sum of a `GdotPdot` upto `n` terms `(ngeq3),` then prove that `(1-r)(d P_n)/(d r)=(1-n)P_n+n P_(n-1),` where `r` is the common ratio of `GdotPdot`

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