Q10 of 104 Page 457

How many terms of the series 2 + 6 + 18 + …. + must be taken to make the sum equal to 728?

Sum of a G.P. series is represented by the formula, , when r>1. ‘Sn’ represents the sum of the G.P. series upto nth terms, ‘a’ represents the first term, ‘r’ represents the common ratio and ‘n’ represents the number of terms.


Here,


a = 2


r = (ratio between the n term and n-1 term) 6 ÷ 2 = 3


Sn = 728


∴


⇒


⇒


⇒


⇒


⇒


∴


∴ 6 terms must be taken to reach the desired answer.


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