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Question
The nth term of a G.P. is 128 and the sum of its n terms is 255. If its common ratio is 2, then its first term is ______.
Options
1
3
8
none of these
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Solution
The nth term of a G.P. is 128 and the sum of its n terms is 255. If its common ratio is 2, then its first term is 1.
Explanation:
Tn = arn-1 = 128 ...(1)
`S_n = (a(r^n-1))/(r-1)` ...(2)
`=> (128r-a)/(r-1) = 255`
Put r = 2
a = 1
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