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In the summation of series of terms in GP when to use : S = a(r n -1)/(r-1) and S = a(1-r n )/(1-r)

 In the summation of series of terms in GP when to use : S = a(rn-1)/(r-1) and S = a(1-rn)/(1-r)

Grade:12

2 Answers

Ashwin Muralidharan IIT Madras
290 Points
9 years ago

Hi Suhas,

 

Both are one and the same. You can use either one of those. Except for the case when r=1 (when the series is not a GP).

 

Regards,

Ashwin (IIT MadraS).

Swapnil Saxena
102 Points
9 years ago

Both are same, so it doesnt really matter . u can use any one of them in any type of question. however mostly we use a(1-rn)/(1-r) when r is fractional and less than 1 ,and a(rn-1)/(r-1) when r is integral or more than 1.

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