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Intern  Joined: 06 Sep 2012
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If x and y are positive integers, what is the GCF of x and y  [#permalink]

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Difficulty:   55% (hard)

Question Stats: 60% (01:38) correct 40% (01:43) wrong based on 582 sessions

### HideShow timer Statistics If x and y are positive integers, what is the GCF of x and y?

(1) When x is divided by y, the remainder is 1.
(2) x^2 - 2xy + y^2 = 1
Math Expert V
Joined: 02 Sep 2009
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Re: If x and y are positive integers, what is the GCF of x and y  [#permalink]

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7
11
If x and y are positive integers, what is the GCF of x and y?

(1) When x is divided by y, the remainder is 1 --> x is one more than some multiple of y (x=yq+1). This implies that x and that multiple are consecutive integer. Now, two consecutive integers are co-prime, which means that they don't share ANY common factor but 1 (for example 20 and 21 are consecutive integers, thus only common factor they share is 1). So, we have that x and yq do not share any common factor but 1, thus x and y also do not share any common factor but 1, which means that the greatest common factor of x and y is 1. Sufficient.

(2) x^2 - 2xy + y^2 = 1 --> (x-y)^2=1 --> |x-y|=1 --> x and y are consecutive integers --> the greatest common factor of x and y is 1. Sufficient.

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Re: If x and y are positive integers, what is the GCF of x and y  [#permalink]

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Bunuel wrote:
If x and y are positive integers, what is the GCF of x and y?

(1) When x is divided by y, the remainder is 1 --> x is one more than some multiple of y (x=yq+1). This implies that x and that multiple are consecutive integer. Now, two consecutive integers are co-prime, which means that they don't share ANY common factor but 1 (for example 20 and 21 are consecutive integers, thus only common factor they share is 1). So, we have that x and yq do not share any common factor but 1, thus x and y also do not share any common factor but 1, which means that the greatest common factor of x and y is 1. Sufficient.

(2) x^2 - 2xy + y^2 = 1 --> (x-y)^2=1 --> |x-y|=1 --> x and y are consecutive integers --> the greatest common factor of x and y is 1. Sufficient.

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sorry, can you please elaborate on the red part? how did you solve (x-y)^2 to get |x-y|=1?
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Re: If x and y are positive integers, what is the GCF of x and y  [#permalink]

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1
JJ2014 wrote:
Bunuel wrote:
If x and y are positive integers, what is the GCF of x and y?

(1) When x is divided by y, the remainder is 1 --> x is one more than some multiple of y (x=yq+1). This implies that x and that multiple are consecutive integer. Now, two consecutive integers are co-prime, which means that they don't share ANY common factor but 1 (for example 20 and 21 are consecutive integers, thus only common factor they share is 1). So, we have that x and yq do not share any common factor but 1, thus x and y also do not share any common factor but 1, which means that the greatest common factor of x and y is 1. Sufficient.

(2) x^2 - 2xy + y^2 = 1 --> (x-y)^2=1 --> |x-y|=1 --> x and y are consecutive integers --> the greatest common factor of x and y is 1. Sufficient.

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sorry, can you please elaborate on the red part? how did you solve (x-y)^2 to get |x-y|=1?

Take the square root from (x-y)^2=1 to get |x-y|=1.
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Re: If x and y are positive integers, what is the GCF of x and y  [#permalink]

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I understand why the answer is D. However, in the first statement, the number x and y dont need to be consecutive numbers to get a remainder 1. for eg 4,9. The GCF is still 1 though.
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Re: If x and y are positive integers, what is the GCF of x and y  [#permalink]

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1
I understand why the answer is D. However, in the first statement, the number x and y dont need to be consecutive numbers to get a remainder 1. for eg 4,9. The GCF is still 1 though.

You are correct but what bunuel said was also correct. He stated

x & ky are consecutive integers not x & y. in this case 9 & 8 (multiple of 4).

8 & 9 have GCF 1 then 4 & 9 can not have GCF greater than 1.

Does this help clarify ?
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Re: If x and y are positive integers, what is the GCF of x and y  [#permalink]

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|x-y|=1 = Does GCF can be -1.
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Re: If x and y are positive integers, what is the GCF of x and y  [#permalink]

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Rajkiranmareedu wrote:
|x-y|=1 = Does GCF can be -1.

By definition, HCF of two or more integers is the largest positive integer which divides them without any remainder. For example, HCF of -4 and -2 is +2.
Nonetheless, in the given problem, it is mentioned that both x and y are positive. Thus, the GCF IS anyways positive.
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GRE 1: Q169 V154 Re: If x and y are positive integers, what is the GCF of x and y  [#permalink]

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Tremendous Question
Here We can see that statement 2 is off course sufficient as two consecutive integers will be co primes
statement 1 is sufficient one number is a multiple and the other is a non multiple of y hence no factors in common.
Hence D
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Re: If x and y are positive integers, what is the GCF of x and y  [#permalink]

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_________________ Re: If x and y are positive integers, what is the GCF of x and y   [#permalink] 09 Jan 2019, 03:05
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