What is the value of x^3 - y^3 ? : GMAT Data Sufficiency (DS)
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What is the value of x^3 - y^3 ?

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What is the value of x^3 - y^3 ? [#permalink]

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10 Oct 2010, 02:46
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What is the value of x^3 - y^3 ?

(1) x^6 - y^6 = 0
(2) y = 0
[Reveal] Spoiler: OA
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10 Oct 2010, 02:54
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vivaslluis wrote:
Hi,

I have a doubt with the following question:
- What is the value of x^3 - y^3 ?
(1) x^6 - y^6 = 0
(2) y = 0

I discard A, B and D because there's not enough information. I know the answer is C, but I don't get a clue of why.

Could someone help me with that?

Thank you!

$$x^6-y^6=0$$ and $$y=0$$
So $$x^6-0^6=0$$
So $$x=0$$
So $$x^3-y^3=0$$
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10 Oct 2010, 10:12
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vivaslluis wrote:
Hi,

I have a doubt with the following question: - What is the value of x^3 - y^3 ?
(1) x^6 - y^6 = 0
(2) y = 0

I discard A, B and D because there's not enough information. I know the answer is C, but I don't get a clue of why.

Could someone help me with that?

Thank you!

So you need $$x^3 - y^3$$.

Statement 1:

$$x^6 - y^6 = (x^3 - y^3)(x^3+y^3) = 0$$

Insufficient, since either one of the two terms could be zero.

Statement 2:

$$y^3 = 0$$, which means $$x^3 - y^3 = x^3.$$ Still insufficient.

Combining both, and substituting y = 0 in the second statement, we get : $$x^6 = 0$$, which means $$x = 0$$, and hence $$x^3 - y^3 = 0$$

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10 Oct 2010, 13:47
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Crystal clear. Thank you!
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23 Dec 2010, 08:06
I disagree with explanations given above. Here is my approach:

Stmt (1) indicates that x=y otherwise x^6-y^6=0 does not hold true. Hence, x^3-y^3 is always equal to zero. SUFF.

Stmt (2) is clearly INSUFF because there is no inf. about x.

If my approach is wrong, I would appreciate any other explanations.
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23 Dec 2010, 08:30
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feruz77 wrote:
I disagree with explanations given above. Here is my approach:

Stmt (1) indicates that x=y otherwise x^6-y^6=0 does not hold true. Hence, x^3-y^3 is always equal to zero. SUFF.

Stmt (2) is clearly INSUFF because there is no inf. about x.

If my approach is wrong, I would appreciate any other explanations.

Yes, your approach is wrong: $$x^6-y^6=0$$ implies that either $$x=y$$ or $$x=-y$$, for example $$1^6-1^6=0$$ and also $$1^6-(-1)^6=0$$. To see this algebraically you could rewrite $$x^6-y^6=0$$ as $$(x^3-y^3)(x^3+y^3)=0$$ --> either $$x^3=y^3$$, or $$x^3=-y^3$$ --> so either $$x=y$$ or $$x=-y$$. OR $$x^6-y^6=0$$ --> $$x^6=y^6$$ --> $$x^2=y^2$$ --> $$x=y$$ or $$x=-y$$.

Now, if $$x=y$$ then $$x^3-y^3=0$$ for any values of $$x$$ and $$y$$ BUT of $$x=-y$$ then $$x^3-y^3=2x^3$$ and we need the value of $$x$$ (or $$y$$) the get the single numerical value of $$2x^3$$. So statement (1) is not sufficient.

(2) y=0 --> clearly insufficient.

(1)+(2) $$y=0$$, so $$x=0$$ too (as $$x^6-y^6=0$$) and $$x^3-y^3=0$$. Sufficient.

Hope it's clear.
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24 Dec 2010, 03:46
This is not tough question, but you need to be carefull not to fall into trap of ODD POWER of numbers. you always need to be suspicious when you see one.
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Re: What is the value of x^3 - y^3 ? [#permalink]

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27 Oct 2015, 06:48
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Re: What is the value of x^3 - y^3 ? [#permalink]

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30 Nov 2015, 14:33
HI All,

This DS question can be solved by TESTing VALUES.

We're asked for the value of X^3 - Y^3.

1) X^6 - Y^6 = 0

IF...
X = 0
Y = 0
0^3 - 0^3 = 0 and the answer is 0.

IF...
X = 1
Y = -1
1^3 - (-1)^3 = 2 and the answer is 2.
Fact 1 is INSUFFICIENT

2) Y = 0

This tells us NOTHING about the value of X, so there's no way to answer the question.
Fact 2 is INSUFFICIENT

Combined, we know...
X^6 - Y^6 = 0
Y = 0

Substituting in the value of Y, we have... X^6 = 0, so X MUST be 0. By extension, the answer to the question is ALWAYS 0^3 - 0^3 = 0.
Combined, SUFFICIENT.

[Reveal] Spoiler:
C

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What is the value of x^3 - y^3 ? [#permalink]

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30 Nov 2015, 17:45
1- xˆ6 - yˆ6 = 0
As we have an even number (6) it means that X and Y can be either negative or positive.
For example.
X=3
Y=-3
If we put this values on the equations, then its going to be valid (equal 0)

However, if we put the same numbers on x^3 - yˆ3 we are going to have:
27 - ( -27) = 54

Therefore number 1 is not valid.

2-> This information alone doesn't help anything, we don't know about the X.

If we use both of them together, we are going to know that the only value will be 0. Therefore C
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Re: What is the value of x^3 - y^3 ? [#permalink]

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18 Sep 2016, 09:07
C is correct. Here's why:

(1) x^6 - y^6 = 0
= (x^3 - y^3)(x^3 + y^3) = 0

INSUFFICIENT - we have no way to know what x^3 - y^3 is equal to

(2) y = 0

INSUFFICIENT - if we plug this into the orig eq we get x^6 = ?

Together - (1) + (2) = if y = 0, then we know x has to equal zero as well in order to make x^6 - y^6 = 0 a true statement
Re: What is the value of x^3 - y^3 ?   [#permalink] 18 Sep 2016, 09:07
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