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If x and y are positive numbers, is x > y? [#permalink]
adiagr wrote:
Bunuel wrote:
If x and y are positive numbers, is x > y?

Combining Statement 1 and 2

\(x^3 < y\)

x < \(y^3\)
Since x is positive we can cube it. so \(x^3 < y^6\)

Now \(x^3 < y\) and \(x^3 < y^6\)

This means that we can take \(x^3\) < \(y\)

This is nothing but Statement (1) only. We know it was insufficient.

So even after combining - insufficient

E is the answer.


I am not completely sure about the above. May be I am wrong, but can't we simply multiply the inequalities since we know that they are positive numbers?

\(x^3 < y\)

x < \(y^3\)

we get: \(x^4\) < \(y^4\)

Hence, x < y and Hence answer should be C and not E

Is there something wrong with this logic?
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If x and y are positive numbers, is x > y? [#permalink]
rockstar23 wrote:
adiagr wrote:
Bunuel wrote:
If x and y are positive numbers, is x > y?

Combining Statement 1 and 2

\(x^3 < y\)

x < \(y^3\)
Since x is positive we can cube it. so \(x^3 < y^6\)

Now \(x^3 < y\) and \(x^3 < y^6\)

This means that we can take \(x^3\) < \(y\)

This is nothing but Statement (1) only. We know it was insufficient.

So even after combining - insufficient

E is the answer.


I am not completely sure about the above. May be I am wrong, but can't we simply multiply the inequalities since we know that they are positive numbers?

\(x^3 < y\)

x < \(y^3\)

we get: \(x^4\) < \(y^4\)

Hence, x < y and Hence answer should be C and not E

Is there something wrong with this logic?


Hi - yes, your logic is not correct. If you have an even exponent it masks the sign of the variable, for example:

if x = -1 and y = -2 then x>y
if x = -2 and y = 3 then y>x

(edit b/c I had it backwards ;D)
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Re: If x and y are positive numbers, is x > y? [#permalink]
Bunuel wrote:
If x and y are positive numbers, is x > y?

(1) \(\frac{x^3}{y} < 1\)
(2) \(\frac{\sqrt[3]{x}}{y} < 1\)


Rule:
[Positive Integer]ˆ(positive integer) gets LARGER, and raised to anything else it gets smaller.
[Negative Integer]ˆ(positive odd integer or positive: odd-numerator/odd-denominator) gets SMALLER, and raised to anything else it gets larger or is undefined.
[Positive ProperFraction]ˆ(positive integer) gets SMALLER, and raised to anything else it gets larger.
[Negative ProperFraction]ˆ(positive integer or negative even) gets LARGER, and raised to anything else it gets smaller or is undefined.


Given: x and y are >0, is x>y?

(1) \(\frac{x^3}{y} < 1\): then xˆ3<y…
if x=1, xˆ3=1 and y=2, then x<y;
if x=0.5, xˆ3=0.125 and y=0.2, then x>y; insufficient.
(2) \(\frac{\sqrt[3]{x}}{y} < 1\): then xˆ(1/3)<y…
if x=1, xˆ(1/3)=1 and y=2, then x<y;
if x=2, xˆ(1/3)=1.25… and y=1.5, x>y; insufficient.

Answer (E).
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Re: If x and y are positive numbers, is x > y? [#permalink]
VeritasKarishma wrote:
Bunuel wrote:
If x and y are positive numbers, is x > y?

(1) \(\frac{x^3}{y} < 1\)

(2) \(\frac{\sqrt[3]{x}}{y} < 1\)


x and y are positive. So we can multiply the inequality by x or y.

(1) \(\frac{x^3}{y} < 1\)
\(x^3 < y\)

How do cubes behave on the right of 0? If x lies between 0 and 1, the cube will be less than x. If x is greater than 1, x^3 will be more than x. We don't know whether x lies in between 0 and 1 or is greater than 1. Not sufficient.

(2) \(\frac{\sqrt[3]{x}}{y} < 1\)
\(\sqrt[3]{x} < y\)

How do cube roots behave on the right of 0? If x lies between 0 and 1, the cube root will be more than x. If x is greater than 1, it will be less than x. We don't know whether x lies in between 0 and 1 or is greater than 1. Not sufficient.

Using both,
x will lie between x^3 and \(\sqrt[3]{x}\) in either case. Since y is greater than both, y must be greater than x. Sufficient.

Answer (C)


Hi VeritasKarishma
Can you please elaborate on the last part for the answer to be C? Thanks.
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Re: If x and y are positive numbers, is x > y? [#permalink]
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