hi guys,
i came across this PS while taking gmat prep 1
For every integer K from 1 to 10, inclusive the kth term of a certain sequence is given by (-1)^(k+1) (1/2^K).
If T is the sum of first 10 terms in the sequence, then T is
A. Greater than 2
B. Between 1 and 2
C. Between 1/2 to 1
D. Between 1/4 to1/2
E. Less than ¼
Gmat prep problem- 1
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- GMATGuruNY
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We don't need to determine the exact sum. Compute only as much as is necessary to see the pattern.sugmomo wrote:hi guys,
i came across this PS while taking gmat prep 1
For every integer K from 1 to 10, inclusive the kth term of a certain sequence is given by (-1)^(k+1) (1/2^K).
If T is the sum of first 10 terms in the sequence, then T is
A. Greater than 2
B. Between 1 and 2
C. Between 1/2 to 1
D. Between 1/4 to1/2
E. Less than ¼
If k=1, -1^(1+1)*(1/2*1) = 1/2
If k=2, -1^(2+1)*(1/2*2) = -1/4
Sum of the first two terms is 1/2 + ( -1/4) = 1/4.
If k=3, -1^(3+1)*(1/2*3) = 1/8.
If k=4, -1^(4+1)*(1/2*4) = -1/16
Now we can see the pattern.
The sum increases by a fraction (1/8, for example) and then decreases by a fraction 1/2 the size (-1/16, for example).
In other words, the sum will alternate between going up a little and then down a little less than it went up.
The sum of the first 2 terms is 1/4. From there, the sum will increase by 1/8, decrease by a smaller fraction (1/16), increase by an even smaller fraction (1/32), and so on. Since all of the fractions after the first two terms will be less than 1/4, the sum will end up somewhere between 1/4 and 1/2.
The correct answer is D.
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Followed here and elsewhere by over 1900 test-takers.
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As a tutor, I don't simply teach you how I would approach problems.
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- goyalsau
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We will have a series
1/2 - 1/4 + 1/8 - 1/16 + 1/32 - 1/ 64 ........................... - 1/1024
we can write it like this
1 - 1/2 + 1/2 - 3/4 + 3/4 - 5/8 + 5/8 - 11/ 16 .......... Last two terms will be 341 / 512 - 681 / 1024
Every thing will cancel out
in the end we will have 1 - 681/1024
it will be 343/1024
and it is indeed between 1/4 and 1/2 .. ( Don't try this at home )
Guru approach is far much more better...
You are great guru........
1/2 - 1/4 + 1/8 - 1/16 + 1/32 - 1/ 64 ........................... - 1/1024
we can write it like this
1 - 1/2 + 1/2 - 3/4 + 3/4 - 5/8 + 5/8 - 11/ 16 .......... Last two terms will be 341 / 512 - 681 / 1024
Every thing will cancel out
in the end we will have 1 - 681/1024
it will be 343/1024
and it is indeed between 1/4 and 1/2 .. ( Don't try this at home )
Guru approach is far much more better...
You are great guru........
Saurabh Goyal
[email protected]
-------------------------
EveryBody Wants to Win But Nobody wants to prepare for Win.
[email protected]
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EveryBody Wants to Win But Nobody wants to prepare for Win.
- goyalsau
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In such series , when there is some pattern ( like in this one we have the power of 2 in the base with increasing order ) like in this one.sugmomo wrote:Thanks Guru
Thanks goyalsau. Can you please elloborate how you arrived to the following step
1 - 1/2 + 1/2 - 3/4 + 3/4 - 5/8 + 5/8 - 11/ 16 .......... Last two terms will be 341 / 512 - 681 / 1024
Thanks,
sugmomo
1/2 - 1/4 + 1/8 - 1/16 + 1/32 - 1/ 64 ........................... - 1/1024
2^1 , 2 ^ 2 , 2 ^ 3 ............ 2 ^ 10
try to form a combination in which you left with two terms. ( i am sorry Buddy Even i don't have clear understanding of this Sometimes i able to do it, Sometimes i d'ont, Like in this one i able to get the first two three terms of the series but then just by looking at the first two three terms you can formulate the last term but i was not able to do it, That's why i have to write all the terms. )
IF Guru can give us some technique then it will be helpful for both of us.
Saurabh Goyal
[email protected]
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EveryBody Wants to Win But Nobody wants to prepare for Win.
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EveryBody Wants to Win But Nobody wants to prepare for Win.