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Re: X and Y. which is true? [#permalink]
05 Sep 2009, 10:31

1

This post received KUDOS

ans is b...... since x and y are both +ive int, x*y>1... only exception being x=y=1 as it is not given they are different integers... however it is given1/x +1/y<2.. this cannot be true if x=y=1.... so one or both have to be > 1

Re: X and Y. which is true? [#permalink]
05 Sep 2011, 13:16

Quote:

X and Y are positive integers. If 1/X + 1/Y < 2, which of the following must be true?

(A) X+Y>4 (B) X*Y>1 (C) X/Y+Y/X<1 (D) (X-Y)^2>0 (E) None of the above

Let X=1, 1+1/Y<2 1/Y<1 1<Y

Y>1 when X=1, A --> yes and no B --> yes C--> yes and no D--> yes and no

Answer: B _________________

"The best day of your life is the one on which you decide your life is your own. No apologies or excuses. No one to lean on, rely on, or blame. The gift is yours - it is an amazing journey - and you alone are responsible for the quality of it. This is the day your life really begins." - Bob Moawab

Re: X and Y. which is true? [#permalink]
05 Sep 2011, 22:20

7

This post received KUDOS

Expert's post

4

This post was BOOKMARKED

barakhaiev wrote:

X and Y are positive integers. If 1/X + 1/Y < 2, which of the following must be true?

(A) X+Y>4 (B) X*Y>1 (C) X/Y+Y/X<1 (D) (X-Y)^2>0 (E) None of the above

Trying a few values makes us realize that the only relation that holds is (B). But how can we be sure that (B) holds for all acceptable values of X and Y.

1/X + 1/Y < 2 implies (1/X + 1/Y)/2 < 1 A useful property of positive numbers is AM >= GM Arithmetic Mean >= Geometric Mean

Say, the numbers are 1/X and 1/Y AM = (1/X + 1/Y)/2 It is given that (1/X + 1/Y)/2 < 1 so we know that AM < 1

GM = \(\sqrt{\frac{1}{X}*\frac{1}{Y}}\)

Since GM <= AM,

\(\sqrt{\frac{1}{X}*\frac{1}{Y}}\) < 1

\(\frac{1}{XY} < 1\) (Squaring the inequality)

\(XY > 1\) (X and Y are positive so the inequality doesn't change) _________________

Re: X and Y. which is true? [#permalink]
07 Sep 2011, 05:05

1

This post received KUDOS

Guys, can you guide me how D is not true? coz last time i checked, square of any number is greater than 0. Even if x is less than y, still, it's square would me more than 0..unless, x = y... _________________

Appreciation in KUDOS please! Knewton Free Test 10/03 - 710 (49/37) Princeton Free Test 10/08 - 610 (44/31) Kaplan Test 1- 10/10 - 630 Veritas Prep- 10/11 - 630 (42/37) MGMAT 1 - 10/12 - 680 (45/34)

Re: X and Y. which is true? [#permalink]
07 Sep 2011, 06:54

Expert's post

krishnasty wrote:

Guys, can you guide me how D is not true? coz last time i checked, square of any number is greater than 0. Even if x is less than y, still, it's square would me more than 0..unless, x = y...

Given: 1/X + 1/Y < 2 Say X = 2, Y = 2 These values satisfy the inequality: 1/2 + 1/2 < 2

But they do not satisfy (D) (X-Y)^2>0 (2-2)^2 = 0, not greater than 0 Hence (D) must not be true for all values. There are values that satisfy the inequality but does not satisfy (D) _________________

Re: X and Y. which is true? [#permalink]
07 Sep 2011, 07:03

Karishma, now i need a confirmation on GMAT questions... lets say that if two unknowns are given (like X and Y ), can we assume that these two are equals? I thought if we say x and y, they are implicitly different numbers..

VeritasPrepKarishma wrote:

krishnasty wrote:

Guys, can you guide me how D is not true? coz last time i checked, square of any number is greater than 0. Even if x is less than y, still, it's square would me more than 0..unless, x = y...

Given: 1/X + 1/Y < 2 Say X = 2, Y = 2 These values satisfy the inequality: 1/2 + 1/2 < 2

But they do not satisfy (D) (X-Y)^2>0 (2-2)^2 = 0, not greater than 0 Hence (D) must not be true for all values. There are values that satisfy the inequality but does not satisfy (D)

_________________

Appreciation in KUDOS please! Knewton Free Test 10/03 - 710 (49/37) Princeton Free Test 10/08 - 610 (44/31) Kaplan Test 1- 10/10 - 630 Veritas Prep- 10/11 - 630 (42/37) MGMAT 1 - 10/12 - 680 (45/34)

Re: X and Y. which is true? [#permalink]
07 Sep 2011, 19:35

Expert's post

krishnasty wrote:

Karishma, now i need a confirmation on GMAT questions... lets say that if two unknowns are given (like X and Y ), can we assume that these two are equals? I thought if we say x and y, they are implicitly different numbers..

VeritasPrepKarishma wrote:

krishnasty wrote:

Guys, can you guide me how D is not true? coz last time i checked, square of any number is greater than 0. Even if x is less than y, still, it's square would me more than 0..unless, x = y...

Given: 1/X + 1/Y < 2 Say X = 2, Y = 2 These values satisfy the inequality: 1/2 + 1/2 < 2

But they do not satisfy (D) (X-Y)^2>0 (2-2)^2 = 0, not greater than 0 Hence (D) must not be true for all values. There are values that satisfy the inequality but does not satisfy (D)

Until and unless they mention 'distinct numbers' or 'X not equal to Y', X and Y can be equal. The equality can be a deal breaker/maker sometimes so you have to make sure that you have analyzed its effects too. _________________

Re: X and Y. which is true? [#permalink]
07 Sep 2011, 20:38

Thanks Karishma for the information.

Quote:

Until and unless they mention 'distinct numbers' or 'X not equal to Y', X and Y can be equal. The equality can be a deal breaker/maker sometimes so you have to make sure that you have analyzed its effects too.

_________________

Appreciation in KUDOS please! Knewton Free Test 10/03 - 710 (49/37) Princeton Free Test 10/08 - 610 (44/31) Kaplan Test 1- 10/10 - 630 Veritas Prep- 10/11 - 630 (42/37) MGMAT 1 - 10/12 - 680 (45/34)

Re: x and y are positive integers. If 1/x + 1/y < 2, which of [#permalink]
19 Apr 2014, 23:11

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Re: x and y are positive integers. If 1/x + 1/y < 2, which of [#permalink]
16 Aug 2015, 16:13

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________

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