DS: co ordinate

[tanyatomar]

if ab is not = 0 and point (-a,b) and (-b,a) are in same quadrant of the xy plane , is the point (-x,y) in this same quadrant??

(1) xy>0
(2) ax>0

OA: c

i selected answer B... thinking that if ax>0 => signs of a and x are same, therefore -a and -x shud point out to same side of origin for the x coordinate of the point. However now i think that confirming xy>0 is also needed because if x and y are not of same sign then even if x lies in same side of x axis (either left or right of origin) but if y is of different sign then point cannot be in the same quadrant as (-a,b)...
please let me know if my way of thinking is correct .. or if there is any other way of looking at this problem..

Tanya

[vijay.gmat2011]

you are right. Having (-a, b) and (-b, a) => both a and b should be of same sign. If both positive, II quad else IV quad

A) xy >0 - Insufficient. Though even (-x, y) will either belong to II or IV quad, it could be opposite to where (-a,b) or (-b,a)

B) ax >0 - Insufficient. Though we know the sign of x, we cannot confirm which quadrant (-x, y) will belong to due to dependency on y

A + B - Sufficient

[RonPurewal]

please search the forum for existing threads before posting!
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here is a very long and exhaustive post on this one:
post10305.html#p10305

this thread is now locked; if you have questions, please post them on the linked thread.