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A positive integer is called "square-Free" if it has no fact

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A positive integer is called "square-Free" if it has no fact [#permalink]

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New post 10 Sep 2013, 00:25
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A positive integer is called "square-Free" if it has no factor that is the square of an integer greater than 1. If n is an even square-free integer, which of the following must also be square free?

A. \(\frac{n}{2}\)
B. \(2n\)
C. \(n + 2\)
D. \(n^2\)
E. none of the above

Spoiler: ::
What are examples of square-free numbers?
Spoiler: OA

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Re: A positive integer is called "square-Free" if it has no fact [#permalink]

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New post 10 Sep 2013, 00:44
fozzzy wrote:
A positive integer is called "square-Free" if it has no factor that is the square of an integer greater than 1. If n is an even square-free integer, which of the following must also be square free?

A. \(\frac{n}{2}\)
B. \(2n\)
C. \(n + 2\)
D. \(n^2\)
E. none of the above

Spoiler: ::
What are examples of square-free numbers?


Per definition a square-free integer has primes in power of 1. For example 5^3 is NOT square-free because it's a multiple of 5^2.

We are told that n=2k, since n itself is square-free, then k also must be square-free --> n/2=k.

Answer: A.

Or just pick some number for n. Say n=2, then among A, B, C, and D, only A (n/2=1) will be square free.

Hope it's clear.
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Re: A positive integer is called "square-Free" if it has no fact [#permalink]

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New post 25 Sep 2013, 13:26
1
fozzzy wrote:
A positive integer is called "square-free" if it has no factor that is the square of an integer greater than 1. If n is an even square-free integer, which of the following must also be square free?

A. \(\frac{n}{2}\)
B. \(2n\)
C. \(n + 2\)
D. \(n^2\)
E. none of the above

Spoiler: ::
What are examples of square-free numbers?

kanusha wrote:
sir, pls explain this problem

Dear Kanusha,
I'm happy to respond to your p.m. :-)

First of all, let's think about this --- the perfect squares greater than 1 are 4, 9, 16, 25, 36, 49, etc. Any numbers divisible by these would not be square-free, and any number that doesn't not happen to be a multiple of these would be square-free. Notice, in particular, all prime numbers are automatically square-free. The first few square-free numbers would be

1, 2, 3, 5, 6, 7, 10, 11, 13, 14 ,15, 17, 19, 21, 22, 23, 26, 29, 30, ....

The problem tells us that n is an even square-free numbers, so candidate could include 2, 6, 10, 14, 22, 26, 30 --- all of these are even numbers that are NOT divisible by 4, so they are the odd multiples of 2, and not even all of those, because some (such as 18) are divisible by other squares (18 is divisible by 9).

Let's think about these .....
(A) n/2 --- using our example even square-free numbers above, this leads to 1, 3, 5, 7, 11, 13, 15 --- all square free --- hmmmm, this is promising
(B) Any even number time 2 is divisible by 4 --- this always leads to a number divisible by a square, i.e. NOT square-free. This is incorrect.
(C) Any odd multiple of 2, plus two, will equal an even multiple of two --- i.e. a multiple of 4, a number that is NOT square-free. This is incorrect.
(D) n^2 would be even times even, which is always divisible by four, i.e. NOT square-free. This is incorrect.
(E) hmmm. :-|

It's very easy to eliminate (B) & (C) & (D). In order to choose (A) over (E), we have to be sure that (A) works, not just for some examples that we pick, but for all possible even square-free numbers. Here's an argument, involving factors.

Suppose n is an even square-free number. As we have seen, it must be an odd multiple of two. This means n = 2*q, where q is some odd number. Because n has no factors that are squares, this necessarily means that q has no factors that are squares. (If P = R*Q, then any factor of Q or R must be a factor of P.) Now, n/2 = q, and q has no square factors, so if n is square-free, then n/2 = q must be square-free. That's why (A) must be the correct answer.

Does all this make sense?
Mike :-)
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Re: A positive integer is called "square-Free" if it has no fact [#permalink]

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New post 26 Sep 2013, 01:29
These definition questions tend to be tricky. I usually get thrown off by such questions... do you have any questions for practice?
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Re: A positive integer is called "square-Free" if it has no fact [#permalink]

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New post 26 Sep 2013, 02:32
1
fozzzy wrote:
These definition questions tend to be tricky. I usually get thrown off by such questions... do you have any questions for practice?


Similar questions:

A "square-free"integer: a-positive-integer-is-called-square-free-if-it-has-no-fact-159462.html

A “Sophie Germain” prime: a-sophie-germain-prime-is-any-positive-prime-number-p-for-132650.html

A Farey sequence: a-farey-sequence-of-order-n-is-the-sequence-of-fractions-129825.html

A “coprime sequence: an-infi-nite-sequence-of-positive-integers-is-called-a-127496.html

A perfect sequence: an-infinite-sequence-of-positive-integers-is-called-a-127696.html

Twin primes: twin-primes-are-defined-as-prime-numbers-that-can-be-express-128433.html

Length of an integer: for-any-positive-integer-n-n-1-the-length-of-n-is-the-126368.html, for-any-integer-k-1-the-term-length-of-an-integer-108124.html, for-any-positive-integer-n-the-length-of-n-is-defined-as-126740.html

"Rhyming" primes: two-different-primes-may-be-said-to-rhyme-128903.html

The “connection” between integers: the-connection-between-any-two-positive-integers-a-and-b-128360.html
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: A positive integer is called "square-Free" if it has no fact [#permalink]

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New post 30 Sep 2013, 05:27
1
Bunuel wrote:
fozzzy wrote:
These definition questions tend to be tricky. I usually get thrown off by such questions... do you have any questions for practice?


Found several such questions:

A “Sophie Germain” prime: a-sophie-germain-prime-is-any-positive-prime-number-p-for-132650.html

A Farey sequence: a-farey-sequence-of-order-n-is-the-sequence-of-fractions-129825.html

A “coprime sequence: an-infi-nite-sequence-of-positive-integers-is-called-a-127496.html

A perfect sequence: an-infinite-sequence-of-positive-integers-is-called-a-127696.html

Twin primes: twin-primes-are-defined-as-prime-numbers-that-can-be-express-128433.html

Length of an integer: for-any-positive-integer-n-n-1-the-length-of-n-is-the-126368.html, for-any-integer-k-1-the-term-length-of-an-integer-108124.html, for-any-positive-integer-n-the-length-of-n-is-defined-as-126740.html

Hope this helps.


Found two more questions:
"Rhyming" primes: two-different-primes-may-be-said-to-rhyme-128903.html
The “connection” between integers: the-connection-between-any-two-positive-integers-a-and-b-128360.html
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
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Re: A positive integer is called "square-Free" if it has no fact [#permalink]

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New post 30 Sep 2013, 11:46
1
Will not pop into too much:

We will do it by hit and trial:

Since 'n' is an even-square free integers.

Then check values of 'n'

n=2 ..... Can be square free
n=4 ... Cannot be since 4 is a factor of 4
n=6.... Can be square free since 6=3*2*1
n=8... Cannot be


Try n=2,6 in answer options and hence (A) it is
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Re: A positive integer is called "square-Free" if it has no fact [#permalink]

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Re: A positive integer is called "square-Free" if it has no fact   [#permalink] 05 Oct 2017, 11:02
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