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Bunuel
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Bunuel
Official Solution:


(1) \(x^3 \lt y^3\). Note that we can always take an odd root from both sides of an inequality (the same for raising both parts of an inequality to an odd power). Now, if we take 3rd root from \(x^3 \lt y^3\) we'll get \(x \lt y\). Sufficient.

(2) \((x+y)(x-y) \lt 0\). This statement tells that \(x^2-y^2 \lt 0\) or \(x^2 \lt y^2\), which is not sufficient to answer whether \(x \lt y\), consider \(x=1\) and \(y=2\) for an YES answer and \(x=1\) and \(y=-2\) for a NO answer.


Answer: A

Hi Bunuel,

For 2, why can't I write -y<x<y ? Is it because we don't really know the sign of y?

Regards
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Bunuel
Official Solution:


(1) \(x^3 \lt y^3\). Note that we can always take an odd root from both sides of an inequality (the same for raising both parts of an inequality to an odd power). Now, if we take 3rd root from \(x^3 \lt y^3\) we'll get \(x \lt y\). Sufficient.

(2) \((x+y)(x-y) \lt 0\). This statement tells that \(x^2-y^2 \lt 0\) or \(x^2 \lt y^2\), which is not sufficient to answer whether \(x \lt y\), consider \(x=1\) and \(y=2\) for an YES answer and \(x=1\) and \(y=-2\) for a NO answer.


Answer: A

Hi Bunuel,

For 2, why can't I write -y<x<y ? Is it because we don't really know the sign of y?

Regards

\(x^2 \lt y^2\) means that |x| < |y|, so y is further from 0 than x is. We can have the following cases:


-----------0---x---y---
-------x---0-------y---
---y---x---0-----------
---y-------0---x------


As you can see, for the first two cases x < y and for the remaining two cases x > y.
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I've revised the question and solution, incorporating additional details for improved clarity. I trust this makes it more comprehensible.
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