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mun23
Is xy+xy<xy?
(1)x^2/y<0
(2)X^3y^3<(xy)^2

it's an easy question provided basics of inequalities are clear.

question is asking for xy +xy <xy i.e. 2xy <xy i.e. xy < 0
this is possible if a) x is negative and y is positive or b) x is positive and y is negative.

from 1) x^2/y < 0 , as square of something is always positive hence x^2 is +ve , so y has to be -ve
....nothing can be concluded about xy ....hence insufficient
from 2) x^3y^3 < x^2y^2 i.e. x^3y^3 - x^2y^2 < 0
i.e. x^2y^2(xy - 1) < 0 as square of something is always +ve
hence xy - 1 < 0 i.e. xy < 1..............not sufficient as xy can be +ve or -ve
now two situation possible
a) if y +ve then xy < 1
=> x < 1/y ............since y is +ve this means x < +ve no. e.g 0.2 < 1 and -2 < 1
hence x can be +ve or -ve

b) if y -ve then xy <1
=> x >1/y (inequality will change)
since y is -ve this means x > some -ve no. e.g. 0.2 > -1 and -0.2 > -1
hence x can be +ve or -ve.

using both Conditions 1 and 2
i.e. y is -ve....from 1
and case b holds from 2
still x can be +ve or -ve
hence xy can be +ve or -ve

hence Option (E)
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Kris01

Now, 1 gives us x^2/y<0. Since, x^2 is always positive...

Statement 2 gives us x^3*y^3<(xy)^2
Since (xy)^2 is always positive, we can divide the inequality by (xy)^2
jbisht
from 1) x^2/y < 0 , as square of something is always positive hence x^2 is +ve , so y has to be -ve


from 2) x^3y^3 < x^2y^2 i.e. x^3y^3 - x^2y^2 < 0
i.e. x^2y^2(xy - 1) < 0 as square of something is always +ve
hence xy - 1 < 0 i.e. xy < 1..............not sufficient as xy can be +ve or -ve

The red parts above are not correct. Square of a number is always non-negative: \(x^2\geq{0}\) and \((xy)^2\geq{0}\).

For (2) it's not correct to write that xy<1, without mentioning that \(xy\neq{0}\), because if \(xy=0<1\), then \(x^3y^3 =0= x^2y^2\).

Hope it's clear.
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Is xy+xy<xy?

Is \(xy+xy<xy\)? --> cancel xy in both sides: is \(xy<0\)? So, the question basically asks whether x and y have the opposite signs.

(1) x^2/y<0. This statement implies that y is negative and x can be anything but zero. Not sufficient.

(2) x^3y^3<(xy)^2 --> \(x^2y^2(xy-1)<0\) --> this statement implies that \(xy\neq{0}\) and \(xy<1\). So, we cannot say whether \(xy<0\). Not sufficient.

(1)+(2) \(y<0\) and \(xy<1\). If \(x=-\frac{1}{2}\) and \(y=-1\), then the answer is NO but if \(x=1\) and \(y=-1\), then the asnwer is YES. Not sufficient.

Answer: E.

Hope it's clear.


Statement 2 states that x^3*y^3<x^2*y^2; this implies that xy is negative (xy<0) which is sufficient. So I chose answer B.

Can you please explain what am I doing wrong? Thank you for your help.
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Bunuel
Is xy+xy<xy?

Is \(xy+xy<xy\)? --> cancel xy in both sides: is \(xy<0\)? So, the question basically asks whether x and y have the opposite signs.

(1) x^2/y<0. This statement implies that y is negative and x can be anything but zero. Not sufficient.

(2) x^3y^3<(xy)^2 --> \(x^2y^2(xy-1)<0\) --> this statement implies that \(xy\neq{0}\) and \(xy<1\). So, we cannot say whether \(xy<0\). Not sufficient.

(1)+(2) \(y<0\) and \(xy<1\). If \(x=-\frac{1}{2}\) and \(y=-1\), then the answer is NO but if \(x=1\) and \(y=-1\), then the asnwer is YES. Not sufficient.

Answer: E.

Hope it's clear.


Statement 2 states that x^3*y^3<x^2*y^2; this implies that xy is negative (xy<0) which is sufficient. So I chose answer B.

Can you please explain what am I doing wrong? Thank you for your help.

How did you deduce that xy < 0? Please check the highlighted part: (2) gives that xy<1.
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mun23
Is xy + xy < xy ?

(1) x^2/y < 0
(2) x^3y^3 < (xy)^2

Is xy + xy < xy ?
is 2xy < xy
is xy < 0 ? x and y have diff sign ??

stmt-1:
x^2/y < 0

x^2 is positive, so y has to be negative. now that we do not know sign of x, we cant be certain if xy <0 or >0. insuff

stmt-2:
x^3y^3 < (xy)^2
(xy)^2(xy-1)<0
as (xy)^2 will be positive xy-1 has to be negative. so we can write xy<1

so xy can be netween 0 and 1 or less than zero. insuff

combining:

again we dont know sign of x. hence xy could be between 0 and 1 or negative. insuff.
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Bunuel
Is xy+xy<xy?

Is \(xy+xy<xy\)? --> cancel xy in both sides: is \(xy<0\)? So, the question basically asks whether x and y have the opposite signs.

(1) x^2/y<0. This statement implies that y is negative and x can be anything but zero. Not sufficient.

(2) x^3y^3<(xy)^2 --> \(x^2y^2(xy-1)<0\) --> this statement implies that \(xy\neq{0}\) and \(xy<1\). So, we cannot say whether \(xy<0\). Not sufficient.

(1)+(2) \(y<0\) and \(xy<1\). If \(x=-\frac{1}{2}\) and \(y=-1\), then the answer is NO but if \(x=1\) and \(y=-1\), then the asnwer is YES. Not sufficient.

Answer: E.

Hope it's clear.


Statement 2 states that x^3*y^3<x^2*y^2; this implies that xy is negative (xy<0) which is sufficient. So I chose answer B.

Can you please explain what am I doing wrong? Thank you for your help.

How did you deduce that xy < 0? Please check the highlighted part: (2) gives that xy<1.

I assumed that x^3*y^3 < x^2*y^2 is only possible if x*y is negative. I failed to take into account that certain positive fractions also fulfill the criteria. I apologize for my mistake. Thank you for your help.
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Is xy + xy < xy ?

(1) x^2/y < 0
(2) x^3y^3 < (xy)^2

Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

The question \(xy + xy < xy\) is equivalent to \(xy < 0\).
Since we have 2 variables and 0 equation, C is most likely to be the answer.

1) & 2)
\(\frac{x^2}{y} < 0 ⇔ x^2 y < 0\).
This is equivalet to\(x≠0\) and \(y < 0\).

\(x^3 y^3 < (xy)^2 ⇔ x^3 y^3 - x^2 y^2 < 0 ⇔ x^2 y^2 ( xy - 1 ) < 0\)
This is equivalent to \(x≠0\), \(y≠0\) and \(xy < 1\)

\(x = \frac{1}{2}\) and \(y = - \frac{1}{2} ⇒ xy = - \frac{1}{4} < 0\) : Yes
\(x = - \frac{1}{2}\) and \(y = - \frac{1}{2} ⇒ xy = \frac{1}{4} > 0\) : No

The answer is E.

Normally for cases where we need 2 more equations, such as original conditions with 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, we have 1 equation each in both 1) and 2). Therefore C has a high chance of being the answer, which is why we attempt to solve the question using 1) and 2) together. Here, there is 70% chance that C is the answer, while E has 25% chance. These two are the key questions. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer according to DS definition, we solve the question assuming C would be our answer hence using 1) and 2) together. (It saves us time). Obviously there may be cases where the answer is A, B, D or E.
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Hi Experts -- chetan2u, VeritasKarishma, Bunuel, nick1816, GMATBusters

I have done quite a few "MUST BE TRUE" questions.

If xy < 1, then WHAT MUST BE TRUE

a) XY < 0

Isn't this accurate ?

I thought S2 was saying something like this and thus i chose B for this question
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Hi Experts -- chetan2u, VeritasKarishma, Bunuel, nick1816, GMATBusters

I have done quite a few "MUST BE TRUE" questions.

If xy < 1, then WHAT MUST BE TRUE

a) XY < 0

Isn't this accurate ?

I thought S2 was saying something like this and thus i chose B for this question

Yes, this is accurate. If some variable is less than 0, it is certainly less than 1.
But stmnt 2 does not give you that xy < 0. It says
(xy)^3 < (xy)^2
(xy)^3 - (xy)^2 < 0
(xy)^2 * (xy - 1) < 0
So one of these factors must be negative. Since (xy)^2 cannot be negative, (xy - 1) < 0
So xy < 1

What I KNOW is that xy < 1.

Can I say now whether xy is less than 0? No, I don't know. I know that it is less than 1 but it could be 1/2 or 0 or -20 etc. I do not know whether it is less than 0. It may or may not be.
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Bunuel

Can I deduce from this part of the expression:

(xy)^2 (xy-1)<0

that either 0<xy<1 or xy<0 and xy≠0?
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bv8562
Bunuel

Can I deduce from this part of the expression:

(xy)^2 (xy-1)<0

that either 0<xy<1 or xy<-1 and xy≠0?

\((xy)^2 (xy-1)<0 \)

(xy)^2 is the square of a number so it must be positive or 0, so for \((xy)^2 (xy-1)<0 \) to be true, we must have \((xy)^2 \neq 0\) and \(xy-1<0\). Thus, \((xy)^2 (xy-1)<0 \) means \(xy<1\) and \((xy)^2 \neq 0\) .
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Bunuel
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Bunuel

Can I deduce from this part of the expression:

(xy)^2 (xy-1)<0

that either 0<xy<1 or xy<-1 and xy≠0?

\((xy)^2 (xy-1)<0 \)

(xy)^2 is the square of a number so it must be positive or 0, so for \((xy)^2 (xy-1)<0 \) to be true, we must have \((xy)^2 \neq 0\) and \(xy-1<0\). Thus, \((xy)^2 (xy-1)<0 \) means \(xy<1\) and \((xy)^2 \neq 0\) .

Sorry asked the wrong question. This is what I want to know:

Can I deduce from this part of the expression:

(xy)^2 (xy-1)<0

since (xy)^2≠0, therefore xy≠0, so, either 0<xy<1 or xy<0?
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Bunuel
bv8562
Bunuel

Can I deduce from this part of the expression:

(xy)^2 (xy-1)<0

that either 0<xy<1 or xy<-1 and xy≠0?

\((xy)^2 (xy-1)<0 \)

(xy)^2 is the square of a number so it must be positive or 0, so for \((xy)^2 (xy-1)<0 \) to be true, we must have \((xy)^2 \neq 0\) and \(xy-1<0\). Thus, \((xy)^2 (xy-1)<0 \) means \(xy<1\) and \((xy)^2 \neq 0\) .

Sorry asked the wrong question. This is what I want to know:

Can I deduce from this part of the expression:

(xy)^2 (xy-1)<0

since (xy)^2≠0, therefore xy≠0, so, either 0<xy<1 or xy<0?
_________________________
Yes.
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