in Quantitative Aptitude edited by
1,432 views
3 votes
3 votes

​​​​​​The sum of the following infinite series is

\[
2+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+\frac{1}{8}+\frac{1}{9}+\frac{1}{16}+\frac{1}{27}+\cdots
\]

  1. $11 / 3$
  2. $7 / 2$
  3. $13 / 4$
  4. $9 / 2$

in Quantitative Aptitude edited by
by
1.4k views

3 Answers

3 votes
3 votes
We can break the series into two different parts

1+ 1/2 + 1/4 + 1/8 + ........... = 1/(1 - 1/2) = 2
1+ 1/3 + 1/9 + 1/27 + ........... = 1/(1 - 1/3) = 3/2

So the answer is 2+3/2 = 7/2
3 votes
3 votes
Let ,

$S= 2+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+\frac{1}{8}+\frac{1}{9}+\frac{1}{16}+\frac{1}{27}+......$

$S=(1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+.......)+(1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+....)$

$S=\frac{1}{1-\frac{1}{2}}+\frac{1}{1-\frac{1}{3}}$

$S=2+\frac{3}{2}$

$S=\frac{7}{2}$

So correct answer is (B) .
1 vote
1 vote
The given expression seems to represent a series where each term is the reciprocal of a certain positive integer. Let's sum the series:

\[2 + \frac{1}{2} + \frac{1}{3} + \frac{1}{4} + \frac{1}{8} + \frac{1}{9} + \frac{1}{16} + \frac{1}{27} + \ldots\]
This is the infinite series where each term is the reciprocal of successive positive integers with denominators being powers of 2 or 3.

\[2 = \frac{2}{1}\]

So, the series starts with 2. Then,

\[\frac{1}{2} + \frac{1}{4} + \frac{1}{8} + \frac{1}{16} + \ldots\]

is a geometric series with the first term \(\frac{1}{2}\) and the common ratio \(\frac{1}{2}\). The sum of this series is given by:

\[S = \frac{a}{1 - r}\]

where \(a\) is the first term and \(r\) is the common ratio. So,

\[S = \frac{\frac{1}{2}}{1 - \frac{1}{2}} = \frac{\frac{1}{2}}{\frac{1}{2}} = 1\]

Similarly,

\[\frac{1}{3} + \frac{1}{9} + \frac{1}{27} + \ldots\]

is a geometric series with the first term \(\frac{1}{3}\) and the common ratio \(\frac{1}{3}\). The sum of this series is:

\[S = \frac{\frac{1}{3}}{1 - \frac{1}{3}} = \frac{\frac{1}{3}}{\frac{2}{3}} = \frac{1}{2}\]

So, the sum of the series is:

\[2 + 1 + \frac{1}{2} = 3 + \frac{1}{2} = \frac{7}{2}\]

Therefore, the sum of the series is \(\frac{7}{2}\) or 3.5 .
Answer:

Related questions

Quick search syntax
tags tag:apple
author user:martin
title title:apple
content content:apple
exclude -tag:apple
force match +apple
views views:100
score score:10
answers answers:2
is accepted isaccepted:true
is closed isclosed:true