DS

[vineetbatra]

Id X^2 - Y^2 a positive number?

1. X- Y is a positive number

2. X + Y is a positive number


Answer is C.

Not sure how to get there.

Thanks,

Vineet

[mike22629]

This is somewhat of a tricky question:

First thing you should always do with DS is to factor information in answer:

So:
Is x^2>y^2
x^2 - y^2 = (x+y)(x-y)

Now, at first glance you might think that the answer is A, because x-y>0 essentially means that x>y. And if x >y, then x^2 must be greater than y^2 right? Well the answer is not necessarily because x and y could both be fractions.

B is insufficient on its own.

But together, if x+y>0 and x-y>0, then x^2-y^2 must be greater than 0 because it is the product of (x+y)(x-y). Because they are both positive, the answer must be positive.

C is answer.

[vineetbatra]

Thanks Mike, when I was looking for option A i was considering negatives, i.e. -3, -4. This also brings a positive value for X-Y, but I forgot about the fractions.

Have you already got 710 in GMAT? that is a great score, how long did you take to solve this question?

Thanks,

Vineet

[mike22629]

On second look, the answer is C, not A, because y could be negative.


For example, if x-y>0, x= 3, y=-4
so x-y=7

but 3^2 - (-4)^2 = -7

So really you need to find out if lxl > lyl

[mike22629]

Yes I took the test on May 26th of this year, and the question took me less than a minute. With practice you will quickly learn how to solve these questions quickly.

[vineetbatra]

not bad at all, I still have to get into the groove.

Cheers and all the best.

Vineet