It is currently 20 Oct 2017, 17:11

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

If N = (x^a)*(y^b), where x and y are prime numbers, and a

Author Message
SVP
Joined: 28 Dec 2005
Posts: 1545

Kudos [?]: 179 [0], given: 2

If N = (x^a)*(y^b), where x and y are prime numbers, and a [#permalink]

Show Tags

11 Jan 2009, 00:11
00:00

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 1 sessions

HideShow timer Statistics

This topic is locked. If you want to discuss this question please re-post it in the respective forum.

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.

Kudos [?]: 179 [0], given: 2

SVP
Joined: 29 Aug 2007
Posts: 2472

Kudos [?]: 843 [0], given: 19

Show Tags

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..
_________________

Gmat: http://gmatclub.com/forum/everything-you-need-to-prepare-for-the-gmat-revised-77983.html

GT

Kudos [?]: 843 [0], given: 19

Senior Manager
Joined: 02 Nov 2008
Posts: 276

Kudos [?]: 117 [0], given: 2

Show Tags

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.

Kudos [?]: 117 [0], given: 2

Manager
Joined: 12 Oct 2008
Posts: 104

Kudos [?]: 10 [0], given: 0

Show Tags

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?

Kudos [?]: 10 [0], given: 0

SVP
Joined: 29 Aug 2007
Posts: 2472

Kudos [?]: 843 [0], given: 19

Show Tags

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 = 2
ii. 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 = 2
ii: a = 1 and b = 1

Togather: Which options are in both 1 and 2: i. a = 2 and b = 2

Therefore it is C.
_________________

Gmat: http://gmatclub.com/forum/everything-you-need-to-prepare-for-the-gmat-revised-77983.html

GT

Kudos [?]: 843 [0], given: 19

SVP
Joined: 28 Dec 2005
Posts: 1545

Kudos [?]: 179 [0], given: 2

Show Tags

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 ?

Kudos [?]: 179 [0], given: 2

SVP
Joined: 29 Aug 2007
Posts: 2472

Kudos [?]: 843 [0], given: 19

Show Tags

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.
_________________

Gmat: http://gmatclub.com/forum/everything-you-need-to-prepare-for-the-gmat-revised-77983.html

GT

Kudos [?]: 843 [0], given: 19

Senior Manager
Joined: 02 Nov 2008
Posts: 276

Kudos [?]: 117 [0], given: 2

Show Tags

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

Kudos [?]: 117 [0], given: 2

Re: DS: Primes   [#permalink] 11 Jan 2009, 12:17
Display posts from previous: Sort by