A function f(x) defined on the set of whole numbers is such that f(x + y) = f(x) × f(y).
If it is given that f(1) = (1 ÷ 3), find the sum of the following infinite series:
f(0) + f(1) + f(2) + f(3) + ....
1) (1 ÷ 2)
2) (3 ÷ 2)
3) (2 ÷ 3)
4) The given infinite series is not convergent and, hence, the required sum is not a finite number.
OA : 2
Please help...not able to understand this concept..
Function
This topic has expert replies
- krishnasty
- Master | Next Rank: 500 Posts
- Posts: 142
- Joined: Mon Jan 10, 2011 8:03 am
- Thanked: 19 times
-
- Legendary Member
- Posts: 1448
- Joined: Tue May 17, 2011 9:55 am
- Location: India
- Thanked: 375 times
- Followed by:53 members
Hi,
f(x+y) = f(x)*f(y)
Let y = 1
f(x+1) = f(x)*f(1) = (1/3)f(x)
So,f(1) = (1/3)f(0) => 1/3 = (1/3)f(0).
So, f(0) = 1
Every term is (1/3) times the previous term. So, it is an infinite geometric series.
Sum of infinite series with first term 'a' and common ratio 'r' is a/(1-r), where |r| < 1
So, sum of the series is 1/(1-1/3) = 1/(2/3) = 3/2
Hence 2
f(x+y) = f(x)*f(y)
Let y = 1
f(x+1) = f(x)*f(1) = (1/3)f(x)
So,f(1) = (1/3)f(0) => 1/3 = (1/3)f(0).
So, f(0) = 1
Every term is (1/3) times the previous term. So, it is an infinite geometric series.
Sum of infinite series with first term 'a' and common ratio 'r' is a/(1-r), where |r| < 1
So, sum of the series is 1/(1-1/3) = 1/(2/3) = 3/2
Hence 2
Cheers!
Things are not what they appear to be... nor are they otherwise
Things are not what they appear to be... nor are they otherwise
- manpsingh87
- Master | Next Rank: 500 Posts
- Posts: 436
- Joined: Tue Feb 08, 2011 3:07 am
- Thanked: 72 times
- Followed by:6 members
f(1+0)=f(1)*f(0);krishnasty wrote:A function f(x) defined on the set of whole numbers is such that f(x + y) = f(x) × f(y).
If it is given that f(1) = (1 ÷ 3), find the sum of the following infinite series:
f(0) + f(1) + f(2) + f(3) + ....
1) (1 ÷ 2)
2) (3 ÷ 2)
3) (2 ÷ 3)
4) The given infinite series is not convergent and, hence, the required sum is not a finite number.
OA : 2
Please help...not able to understand this concept..
also f(1)=1/3;
therefore; 1/3=1/3*f(0);
f(0)=1;
f(0)+f(1)+f(2)+f(3)+........
f(2)=f(1+1)=1/3*1/3=(1/3)^2;
f(3)=f(2+1)=(1/3)^2*1/3=(1/3)^3;
.
.
.
thus, f(0)+f(1)+f(2)+f(3)+........= 1+1/3+1/3^2+1/3^2+...
which is an infinite g.p. whose common ratio is 1/3 and first term is 1,, therefore its sum is a/1-r; where -1<r<1;
therefore, (1)/(1-1/3)=3/2;
hence B
O Excellence... my search for you is on... you can be far.. but not beyond my reach!
GMAT/MBA Expert
- Ian Stewart
- GMAT Instructor
- Posts: 2621
- Joined: Mon Jun 02, 2008 3:17 am
- Location: Montreal
- Thanked: 1090 times
- Followed by:355 members
- GMAT Score:780
This is miles away from being a realistic GMAT question. You will *never* be tested on sums of infinite series on the GMAT, nor do you need to know what it means for a series to be 'convergent'. If the author of your prep material thinks this is a realistic practice question, he or she doesn't understand the test.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com
ianstewartgmat.com
ianstewartgmat.com
- krishnasty
- Master | Next Rank: 500 Posts
- Posts: 142
- Joined: Mon Jan 10, 2011 8:03 am
- Thanked: 19 times
thnx a bunch Ian..its a relief to know such questions are not a part of GMAT test.Ian Stewart wrote:This is miles away from being a realistic GMAT question. You will *never* be tested on sums of infinite series on the GMAT, nor do you need to know what it means for a series to be 'convergent'. If the author of your prep material thinks this is a realistic practice question, he or she doesn't understand the test.
---------------------------------------
Appreciation in thanks please!!
Appreciation in thanks please!!
- amit2k9
- Master | Next Rank: 500 Posts
- Posts: 461
- Joined: Tue May 10, 2011 9:09 am
- Location: pune
- Thanked: 36 times
- Followed by:3 members
the series is 1+ 1/3+ 1/3^2 + ....
s = a/(1-r) = 1/1-(1/3) = 3/2.
f(0+1) = f(0)*f(1) = f(0)* 1/3 = 1/3
f(0) = 1 then
s = a/(1-r) = 1/1-(1/3) = 3/2.
f(0+1) = f(0)*f(1) = f(0)* 1/3 = 1/3
f(0) = 1 then
For Understanding Sustainability,Green Businesses and Social Entrepreneurship visit -https://aamthoughts.blocked/
(Featured Best Green Site Worldwide-https://bloggers.com/green/popular/page2)
(Featured Best Green Site Worldwide-https://bloggers.com/green/popular/page2)