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If N = (x^a)*(y^b), where x and y are prime numbers, and a [#permalink]
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11 Jan 2009, 00:11
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If N = (x^a)*(y^b), where x and y are prime numbers, and a and b are positive intergers. Is N^1/2 an integer 1. a+b is even 2. a*b is even im disputing the OA for this one; got it off another forum. == Message from GMAT Club Team == This is not a quality discussion. It has been retired. If you would like to discuss this question please repost it in the respective forum. Thank you! To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative  Verbal Please note  we may remove posts that do not follow our posting guidelines. Thank you.



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Re: DS: Primes [#permalink]
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11 Jan 2009, 00:57
pmenon wrote: If N = (x^a)*(y^b), where x and y are prime numbers, and a and b are positive intergers. Is N^1/2 an integer
1. a+b is even 2. a*b is even
im disputing the OA for this one; got it off another forum. C makes sense. 1: a and b both are either odd or even. insuff... 2: if a is odd, b is even and vice versa. if a is even, b is also even. insufff... togather, from 2 at least either a or b is even. from 1 both are either even or odd. suff... both have to be even.......... if a and b are even and they are +ve, they are at leat 2 or its multiple. so sqrtn is an integer. C..
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Re: DS: Primes [#permalink]
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11 Jan 2009, 01:37
GMAT TIGER wrote: pmenon wrote: If N = (x^a)*(y^b), where x and y are prime numbers, and a and b are positive intergers. Is N^1/2 an integer
1. a+b is even 2. a*b is even
im disputing the OA for this one; got it off another forum. C makes sense. 1: a and b both are either odd or even. insuff... 2: if a is odd, b is even and vice versa. if a is even, b is also even. insufff... togather, from 2 at least either a or b is even. from 1 both are either even or odd. suff... both have to be even.......... if a and b are even and they are +ve, they are at leat 2 or its multiple. so sqrtn is an integer. C.. Precisely! Thanks, GMAT Tiger.



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Re: DS: Primes [#permalink]
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11 Jan 2009, 08:44
GMAT TIGER wrote: 1: a and b both are either odd or even. insuff...
2: if a is odd, b is even and vice versa. if a is even, b is also even. insufff...
togather, from 2 at least either a or b is even. from 1 both are either even or odd. suff... both have to be even..........
Could you pls elaborate how you came to the conclusion that both a and b are even. from 2 either a or b is even, in the same way either a or b is odd. From 1 both a and b are either even or odd. How do you come to the conclusion that they both have to be even? It's 50/50... What am I missing?



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Re: DS: Primes [#permalink]
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11 Jan 2009, 09:04
linau1982 wrote: GMAT TIGER wrote: 1: a and b both are either odd or even. insuff...
2: if a is odd, b is even and vice versa. if a is even, b is also even. insufff...
togather, from 2 at least either a or b is even. from 1 both are either even or odd. suff... both have to be even..........
Could you pls elaborate how you came to the conclusion that both a and b are even. from 2 either a or b is even, in the same way either a or b is odd. From 1 both a and b are either even or odd. How do you come to the conclusion that they both have to be even? It's 50/50... What am I missing? From 2:Suppose: even = 2 (or 4, 6, or so on............) and b = 1 (or 3, 5, or so on) i. a = 2 and b = 2ii. a = 2 and b = 1 iii. a = 1 and b = 2 but a = 1 and b = 1 are not possible. From 1: i. a = 2 and b = 2ii: a = 1 and b = 1 Togather: Which options are in both 1 and 2: i. a = 2 and b = 2Therefore it is C. Did I clearify your doubts?
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Re: DS: Primes [#permalink]
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11 Jan 2009, 09:49
so here is my question: from statements 1 and 2, i think the only option left is for both a and b to be even, right ?
if so, then whats to stop x=y=2 and a=b=2 , in which case you have a YES answer. Or, you could have x=y=2 and a=2, b=3, in which case you have a NO answer.
Thats why I answered E. Where am i mistaken ?



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Re: DS: Primes [#permalink]
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11 Jan 2009, 11:00
pmenon wrote: so here is my question: from statements 1 and 2, i think the only option left is for both a and b to be even, right ?
if so, then whats to stop x=y=2 and a=b=2 , in which case you have a YES answer. Or, you could have x=y=2 and a=2, b=3, in which case you have a NO answer.
Thats why I answered E. Where am i mistaken ? The highlighted part is incorrect. if so, then it violets statement 1 that (a+b) = even because 2+3 = 5. What you are missing is either one of a or b must be even, in 2, to have a*b even. If one of a or b is even in 2, then both (a and b) must be even in 1 because in 1 both should be either even or odd. since from 2 either one must be even, ab must be even in 1. Therefore it is C not E.
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Re: DS: Primes [#permalink]
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11 Jan 2009, 12:17
pmenon wrote: If N = (x^a)*(y^b), where x and y are prime numbers, and a and b are positive intergers. Is N^1/2 an integer
1. a+b is even 2. a*b is even
im disputing the OA for this one; got it off another forum. Statement 1 tells us either: 1) A and B are both odd 2) A and B are bot even In order for a+b to be even they must both be either even or odd Statement 2 tells us either: 1) A and B are both even 2) A OR B is even In order for a*b to be even at least one of these variables needs to be even Combining the 2 statements leads to answer C == Message from GMAT Club Team == This is not a quality discussion. It has been retired. If you would like to discuss this question please repost it in the respective forum. Thank you! To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative  Verbal Please note  we may remove posts that do not follow our posting guidelines. Thank you.










