TS ICET 23 May 2019 Shift 2 Paper

Question : 129 of 200

Marks: +1, -0

How many first few terms of the geometric progression 3, 6, 12, 24, ... add up to 765?

Solution:

Here

a=3,r=6\/3=2

Let

S_n=765

⇒{a(r^{n}-1)}/{r-1}=765

⇒{\left.3(2)^{n}-1)}/{2-1}=765

⇒ 2^{n}-1=255

⇒ 2^{n}=256=2^{8}

⇒ n=8

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