Data Sufficiency

kris610 wrote:
B) tells you that a and x have the same sign. This indicates a and -x are of opposite signs, which should be sufficient to say that (-x,y) does not lie in the same quadrant that contains (a,-b) and (b,-a). For example if a is positive, the points (a,-b) and (b,-a) will be in the 4th quadrant, but (-x,y) will be in the 2nd or the 3rd quadrant.

Am I missing something here?

GMATGuruNY wrote:
Question rephrased: Are x and y the same sign, and is that sign different from that of a and b?


Statement 2: ax>0.
Thus, a and x are the same sign.
No information about y.
Insufficient.

djiddish98 wrote:

GMATGuruNY wrote:
Statement 2: ax>0.
Thus, a and x are the same sign.
No information about y.
Insufficient.

If we know that x and a are the same sign, doesn't that prove that the point (-x,y) is NOT in the same quadrant as (a,-b)? Why do we need information about Y?

GMATGuruNY wrote:

raju232007 wrote:If ab is not equal to 0 and points (-a,b) & (-b,a) are in the same quadrant of the xy plane, is the point (-x,y) in the same quadrant?

1.xy>0
2.ax>0

The explanation for this problem has already been posted somewhere in this forum..but i still have my doubts on how to solve this one.ans is C..Kindly help...

This question is from GMATPrep. I've amended the information above to reflect that given in the original question.

Only two cases satisfy the constraint that (-a,b) and (-b,a) are in the same quadrant.

Case 1: Both a and b are positive
Let a=2, b=3.
Then (-a,b) = (-2,3) and (-b,a) = (-3,2).
Both points are in quadrant II.

Case 2: Both a and b are negative
Let a=-2, b=-3.
Then (-a,b) = (2,-3) and (-b,a) = (3,-2).
Both points are in quadrant IV.

Thus, for (-a,b) and (-b,a) to be in the same quadrant, a and b must have the same sign.
Implication:
For (-x,y) to be in the same quadrant as (-a,b) and (-b,a), x and y must have the SAME SIGN as a and b.

Question rephrased: Do x and y have the same sign as a and b?

Statement 1: xy > 0.
Here, x and y have the same sign.
No way to determine whether this sign is the same as that of a and b.
INSUFFICIENT.

Statement 2: ax>0.
Here, a and x have the same sign.
No information about y.
INSUFFICIENT.

Statements 1 and 2 combined:
Since x and y have the same sign, and a and x have the same sign, x, y, and a -- and thus b -- all have the same sign.
SUFFICIENT.

The correct answer is C.

Mo2men wrote:Dear GMATGuru,

If the question asks the following:
is the point (x,y) in the same quadrant? (P.S.: I deleted the sign before x)

Does this make statement A sufficient.

Here, (x,y) will be either in quadrant 1 or 4 which does not match any quadrant of II or IV.

Am I correct?