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In a geometric series, the first term is...

In a geometric series, the first term is `a` and common ratio is `rdot` If `S_n` denotes the sum of the terms and `U_n=sum_(n=1)^n S_n` ,then `r S_n+(1-r)U_n` equals a.`0` b. `n` c. `n a` d. `n a r`

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