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# If sqrt(x) is a positive integer, is sqrt(x) a prime number?

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Intern
Joined: 20 Mar 2006
Posts: 3
If sqrt(x) is a positive integer, is sqrt(x) a prime number? [#permalink]

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23 Mar 2006, 20:41
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If sqrt(x) is a positive integer, is sqrt(x) a prime number?

1: x is divisible by exactly three positive integers
2: all positive factors of x are odd
Director
Joined: 10 Oct 2005
Posts: 718

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23 Mar 2006, 22:46
1 st statement)
x can be 4 since it is divisible by 4,2,1
x can be 9 since it is divisible by 9,3,1
so insuff
2 st)
x can be 9 9,3,1
and 25 5 25 1
and 49 49 7 1
I am not sure but E
_________________

IE IMBA 2010

Manager
Joined: 22 Apr 2005
Posts: 129
Location: Los Angeles

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24 Mar 2006, 00:12
Yurik79 wrote:
1 st statement)
x can be 4 since it is divisible by 4,2,1
x can be 9 since it is divisible by 9,3,1
so insuff
2 st)
x can be 9 9,3,1
and 25 5 25 1
and 49 49 7 1
I am not sure but E

You may not know it but if x=4 - sqrt(4) = 2 - prime number
same with x=9
Manager
Joined: 22 Apr 2005
Posts: 129
Location: Los Angeles

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24 Mar 2006, 00:20
The solution is the following.

1. x is divisible by exactly three positive integers

any integer is divisable by itself and 1. This gives us 2 positive integers.

If there is only one third integer that divides x - it must be a prime number, because otherwise factors of that third integer would also divide x.

So x is divisable by 1, x and some prime.
That means x = (some prime)^n
But if n>2 x must be divisable by (some prime)^(n-1) - contradiction.
So far we found out, that x = (some prime)^2 => sqrt(x) = some prime

Sufficient

2. all positive factors of x are odd

pick x = 5*5*3*3 => sqrt(x) = 5*3 - not a prime number

Insufficient

Director
Joined: 06 Feb 2006
Posts: 897

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24 Mar 2006, 01:35
Should be A...

1) statement... X cannot be more than 10 since the figure can only have 3 multiples....

If x=9, sqrt X is a prime number..... x divisable by 1, 3 and 9

If x=4 sqrt X is also a prime number.... x divisable by 1, 2 and 4

If x=16 sqrt x is not a prime number but x has multiples 1, 2, 4, 8, 16 which cannot be the case according to the statement.

2) statement... insufficient. explanation post above
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5043
Location: Singapore

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24 Mar 2006, 01:54
1) x is divisible by exactly 3 positive integer --> not prime since prime numbers can only be divided by 1 and itself.

2) Not sufficient. Could be prime or not prime.

Ans A
Director
Joined: 06 Feb 2006
Posts: 897

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24 Mar 2006, 01:58
ywilfred wrote:
1) x is divisible by exactly 3 positive integer --> not prime since prime numbers can only be divided by 1 and itself.

2) Not sufficient. Could be prime or not prime.

Ans A

It is amazing how you interpreted the question and still got a correct answer
Manager
Joined: 22 Apr 2005
Posts: 129
Location: Los Angeles

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24 Mar 2006, 10:13
You are both pretty good in that department

>1) statement... X cannot be more than 10 since the figure can only have 3 multiples....

Take 49 and tell me how many factros you got.
Or 12769.

SimaQ wrote:
ywilfred wrote:
1) x is divisible by exactly 3 positive integer --> not prime since prime numbers can only be divided by 1 and itself.

2) Not sufficient. Could be prime or not prime.

Ans A

It is amazing how you interpreted the question and still got a correct answer
24 Mar 2006, 10:13
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