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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