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If x and y are positive integers such that x < y, which of the followi

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If x and y are positive integers such that x < y, which of the followi  [#permalink]

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New post 29 Mar 2018, 00:21
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A
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C
D
E

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If x and y are positive integers such that x < y, which of the following expressions must be less than 1?


I. \(\sqrt{\frac{y}{x}}\)

II. \(\frac{x^2 - 100}{y^2 - 100}\)

III. \(\frac{x}{y}\)


(A) I only
(B) II only
(C) III only
(D) I and II only
(E) II and III only

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Re: If x and y are positive integers such that x < y, which of the followi  [#permalink]

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New post Updated on: 29 Mar 2018, 10:03
Bunuel wrote:
If x and y are positive integers such that x < y, which of the following expressions must be less than 1?


I. \(\sqrt{\frac{y}{x}}\)

II. \(\frac{x^2 - 100}{y^2 - 100}\)

III. \(\frac{x}{y}\)


(A) I only
(B) II only
(C) III only
(D) I and II only
(E) II and III only


As there are specific rules that govern if a fraction is larger or smaller than 1, we'll use them.
This is a Precise approach.

A fraction is smaller than 1 if its numerator is smaller than its denominator.
That is, \(\frac{smaller}{larger}<1\). Since x<y and both are positive, then \(\frac{x}{y}<1\).
III is true so (A), (B), (D) are eliminated.
So all we need to know is if II is true. Since x < y and both are positive integers then x^2 < y^2 meaning that x^2 - 100 < y^2 - 100.
So, if both are positive then \(\frac{x^2 - 100}{y^2 - 100}\) is smaller than 1.
But what happens if both numerator and denominator are negative? In this case, then the numberator is a 'smaller number' = 'larger negative' and the denominator a 'larger number' = 'smaller negative' and once we cancel out the minus sign we get a fraction whose numerator is larger than its denominator, meaning that it is larger than 1.

(C) s our answer.
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Originally posted by DavidTutorexamPAL on 29 Mar 2018, 00:49.
Last edited by DavidTutorexamPAL on 29 Mar 2018, 10:03, edited 1 time in total.
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Re: If x and y are positive integers such that x < y, which of the followi  [#permalink]

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New post 29 Mar 2018, 09:56
DavidTutorexamPAL wrote:
Bunuel wrote:
If x and y are positive integers such that x < y, which of the following expressions must be less than 1?


I. \(\sqrt{\frac{y}{x}}\)

II. \(\frac{x^2 - 100}{y^2 - 100}\)

III. \(\frac{x}{y}\)


(A) I only
(B) II only
(C) III only
(D) I and II only
(E) II and III only


As there are specific rules that govern if a fraction is larger or smaller than 1, we'll use them.
This is a Precise approach.

A fraction is smaller than 1 if its numerator is smaller than its denominator.
That is, \(\frac{smaller}{larger}<1\). Since x<y and both are positive, then \(\frac{x}{y}<1\).
III is true so (A), (B), (D) are eliminated.
So all we need to know is if II is true. Since x < y and both are positive integers then x^2 < y^2 meaning that x^2 - 100 < y^2 - 100.
So, if both are positive then \(\frac{x^2 - 100}{y^2 - 100}\) is smaller than 1.
But what happens if both are negative? In this case, then the numberator is a 'smaller number' = 'larger negative' and the denominator a 'larger number' = 'smaller negative' and once we cancel out the minus sign we get a fraction whose numerator is larger than its denominator, meaning that it is larger than 1.

(C) s our answer.


The question states that x and y are positive integers, hence our answer will be (E)
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Re: If x and y are positive integers such that x < y, which of the followi  [#permalink]

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New post 29 Mar 2018, 10:01
qwertybd wrote:
The question states that x and y are positive integers, hence our answer will be (E)


Try x = 2 and y = 3.
Then:
x^2 - 100 = -96
y^2 - 100 = -91.

Their quotient is 96/91 which is larger than 1.

I think the problem came from lack of clarity in my answer: 'both are negative' doesn't refer to x and y but to the expressions (x^2 - 100) and (y^2 - 100)
I edited the original answer for clarity, thanks!
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Re: If x and y are positive integers such that x < y, which of the followi  [#permalink]

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New post 10 Apr 2018, 22:50
1
Hi, Bunuel sir. we expect your clarification for answer of the above question.
Why the answer shows the option E.
Where, it is easily found that x=2, y=3, so (x^2 - 100)/(y^2 -100) >1.
So ans should be C only
Then based on which cogent logic it should be the answer option E.
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Re: If x and y are positive integers such that x < y, which of the followi  [#permalink]

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New post 10 Apr 2018, 23:41
Jamil1992Mehedi wrote:
Hi, Bunuel sir. we expect your clarification for answer of the above question.
Why the answer shows the option E.
Where, it is easily found that x=2, y=3, so (x^2 - 100)/(y^2 -100) >1.
So ans should be C only
Then based on which cogent logic it should be the answer option E.


Yes, the answer is C. Edited the OA. Thank you.
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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:
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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: If x and y are positive integers such that x < y, which of the followi &nbs [#permalink] 10 Apr 2018, 23:41
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