Q. 11

If the G.P.’s 5, 10, 20, …. And 1280, 640, 320, … have their nth terms equal, find the value of n.

Answer :

GP is given by


a, ar, ar2, , arn-1


In the sequence 5, 10, 20, 40, …


First term, a = 5


Common ratio, r


Equate the term to be found with the nnth term.


arn-1=1280


5 × 2n-1 =1280


2n-1 = 256


n - 1 = 8


n =9


9th term is equal.


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