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# M21-30 Mistake?

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Intern
Joined: 05 Nov 2012
Posts: 4

Kudos [?]: 1 [0], given: 2

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19 Feb 2013, 04:01
1
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Is $$x^2*y^3*z>0?$$

(1) yz>0

(2) xz<0

The official answer from Gmat Club is C.

I strongly think the correct answer is A and here is why:

From statement 1 y & z have the same sign wither both - or both +. This mean that the part of the equation given $$y^3*z$$ is positive. And since $$x^$$ has to be positive we can conclude that statement 1 is sufficient.

What do you think?

Cheers

Kudos [?]: 1 [0], given: 2

Math Expert
Joined: 02 Sep 2009
Posts: 41893

Kudos [?]: 128823 [2], given: 12183

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19 Feb 2013, 04:28
2
KUDOS
Expert's post
wieseljonas wrote:
Is $$x^2*y^3*z>0?$$

(1) yz>0

(2) xz<0

The official answer from Gmat Club is C.

I strongly think the correct answer is A and here is why:

From statement 1 y & z have the same sign wither both - or both +. This mean that the part of the equation given $$y^3*z$$ is positive. And since $$x^$$ has to be positive we can conclude that statement 1 is sufficient.

What do you think?

Cheers

Is $$x^2*y^3*z>0$$?

Inequality $$x^2*y^3*z>0$$ to be true:
1. $$y$$ and $$z$$ must be either both positive or both negative, so they must have the same sign (in this case $$y^3*z$$ will be positive);
AND
2. $$x$$ must not be zero (in this case $$x^2$$ will be positive).

(1) $$yz>0$$. From this statement it follows that $$y$$ and $$z$$ are either both positive or both negative, so the first condition is satisfied. But we don't know about $$x$$ (the second condition). Not sufficient.

(2) $$xz<0$$. From this statement it follows that $$x\neq{0}$$, so the second condition is satisfied. Don't know about the signs of $$y$$ and $$z$$ (the first condition). Not sufficient.

(1)+(2) Both conditions are satisfied. Sufficient.

Similar questions to practice:
is-x-2-y-5-z-0-1-xz-y-0-2-y-z-98341.html
is-x-7-y-2-z-3-0-1-yz-0-2-xz-127692.html
m21-q30-96613.html

Hope it helps.
_________________

Kudos [?]: 128823 [2], given: 12183

Intern
Joined: 05 Nov 2012
Posts: 4

Kudos [?]: 1 [0], given: 2

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19 Feb 2013, 04:30
Ok thanks.

It's the x must not be zero that I missed! Thanks you!

Kudos [?]: 1 [0], given: 2

Re: M21-30 Mistake?   [#permalink] 19 Feb 2013, 04:30
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# M21-30 Mistake?

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