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If x < 0 and 0 < y < 1, which of the following has

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If x < 0 and 0 < y < 1, which of the following has

by mitzwillrockgmat » Thu May 20, 2010 3:44 am
If x < 0 and 0 < y < 1, which of the following has the greatest value?

A. x2
B. (xy)^2
C. (x/y )^2
D. x^2/y
E. x^2y

Yes, i know this is an easy question & the answer is c if you use values x = -1 & y=1/2 but my question is, in less than a minute how can i reach this solution? i'm timing mysle fand i ended up spending 5 minutes plugging in values before reaching this combination!

So should i just simply keep values like -1 , 1/2 etc in mind so that i can plug them in quickly or is there another approach to this?

Also other than -1, 1/2 what are other values are good for testing?

Thank you!!
:)
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by kstv » Thu May 20, 2010 3:53 am
y is a fraction
so 1/y will be > 1

narrow it to C and D
both are multiplied by 1/y

since 1/y²> 1/y
so C
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by indiantiger » Thu May 20, 2010 4:15 pm
given x<0 {first thing check all the answer choices if you see any value that has odd power on x and rest are even powers rule that choice out as it is to maximize the value and y is +ive}

and 0<y<1

A. x^2 , keep A
B.(xy)^2, y is a fraction which will make xy smaller than x (ignoring the sign of x), keep A
C. (x/y)^2, lets say x = -5 and y = 1/4 (x/y) becomes = -20 , (x/y)^2 = 400, keep C, discard A
D. x^2/Y, lets try with the same values x^2 = 25, x^2/y = 25*4 = 100, keep C
E.x^2*y, same as B, Y is a fraction value.

I hope the explanation helps
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by frank1 » Thu May 20, 2010 6:24 pm
My approach...
read and understand question and answers (16 seconds)
x is negative and suppose y is .5 or 1/2 (5 seconds)

now when number is divided by .5 it actually increases and when multiplied it decreses...
so y in deno gives greater value (to think recall this 10 seconds)

scan answers and eliminate some and get to C and D ...15 seconds
here i was in problem
so took x=-2 and y=.5
c gave 16 where as d gave 8 only (20 seconds)

so c...

but still not under 1 minutes....this was best i could do ...that also because other 3 options were not that tricky....
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by Stuart@KaplanGMAT » Thu May 20, 2010 6:38 pm
mitzwillrockgmat wrote:If x < 0 and 0 < y < 1, which of the following has the greatest value?

A. x2
B. (xy)^2
C. (x/y )^2
D. x^2/y
E. x^2y

Yes, i know this is an easy question & the answer is c if you use values x = -1 & y=1/2 but my question is, in less than a minute how can i reach this solution? i'm timing mysle fand i ended up spending 5 minutes plugging in values before reaching this combination!

So should i just simply keep values like -1 , 1/2 etc in mind so that i can plug them in quickly or is there another approach to this?

Also other than -1, 1/2 what are other values are good for testing?

Thank you!!
:)
Hi,

the question is "which of the following has the greatest value" - there's no wriggle room, so no matter which numbers you pick, the same choice will be the biggest.

In other words, you just need to pick one set of numbers and, as long as those numbers are permissible (i.e. follow the rules given), you'll get the right answer to the question.

Since y is a fraction, 1/2 makes sense, since it's usually the easier fraction to work with.

x is negative, so you certainly can pick x=-1 (since that's also a fairly simple number), but you could also pick x=-10 or -37 or -43.228 if you think that's going to make the math simpler.
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by mitzwillrockgmat » Thu May 20, 2010 8:32 pm
Stuart Kovinsky wrote:
mitzwillrockgmat wrote:If x < 0 and 0 < y < 1, which of the following has the greatest value?

A. x2
B. (xy)^2
C. (x/y )^2
D. x^2/y
E. x^2y

Yes, i know this is an easy question & the answer is c if you use values x = -1 & y=1/2 but my question is, in less than a minute how can i reach this solution? i'm timing mysle fand i ended up spending 5 minutes plugging in values before reaching this combination!

So should i just simply keep values like -1 , 1/2 etc in mind so that i can plug them in quickly or is there another approach to this?

Also other than -1, 1/2 what are other values are good for testing?

Thank you!!
:)
Hi,

the question is "which of the following has the greatest value" - there's no wriggle room, so no matter which numbers you pick, the same choice will be the biggest.

In other words, you just need to pick one set of numbers and, as long as those numbers are permissible (i.e. follow the rules given), you'll get the right answer to the question.

Since y is a fraction, 1/2 makes sense, since it's usually the easier fraction to work with.

x is negative, so you certainly can pick x=-1 (since that's also a fairly simple number), but you could also pick x=-10 or -37 or -43.228 if you think that's going to make the math simpler.
thanks all! :)
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by artistocrat » Mon Feb 21, 2011 1:04 pm
Just my two cents but I happened to notice that every single answer choice has x^2, so in light of this x^2 is a constant, and hence, irrelevant. The reason its irrelevant is that even if x is negative, x^2 will always be positive, so it doesn't matter what value you use to plug in for x. It simply doesn't matter. so the question can be rephrased in this way:

(A) 1
(B) y^2
(C) 1/(Y^2)
(D) 1/Y
(E) Y

Clearly C wins.

Good luck!
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