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Which term of the series 5, 10, 20, 40, ...

Which term of the series 5, 10, 20, 40, is 1280 ?

A

10th

B

9th

C

8th

D

None of these

Text Solution

Verified by Experts

The correct Answer is:
B
The given series is a G.P. with first term
a = 5 and coinmon ratio = r = `(10)/(5)=2`
Let 1280 be the nth term
`therefore` nth term = `ar^(n-1)`
or, `1280 = ar^(n-1)`
Put a = 5, r = 2
`1280 = 5. (2)^(n-1)`
or, `256 = (2)^(n-1)`
or, `2^(8) =(2)^(n-1)" "[therefore" ""Equate powers" ]`
or, 8 = n - 1
or, n = [9]
Therefore, required term = 9th
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