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quadrants

by Uri » Wed May 06, 2009 8:12 am
If ab does not equal 0 and points (-a, b) and (-b, a) are in the same quadrant of the xy-plane, is point (-x,y) in this same quadrant?

1) xy > 0
2) ax > 0

This problem has been briefly discussed earlier in this forum. Still it would be great to get some other approaches. So, I am posting again.

OA: [spoiler](C)[/spoiler]

Links to the earlier discussions:
https://www.beatthegmat.com/coordinate-g ... 19732.html
https://www.beatthegmat.com/gmat-prep-co ... 28858.html
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Source: — Data Sufficiency |
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by bluementor » Thu May 07, 2009 12:36 am
first let's examine how (-a, b) and (-b, a) are in the same quadrant. In order for both these points to be in the same quadrant, the following condition must be met:

(the sign of -a = the sign of -b) AND (the sign of b = the sign of a)

here, I am basically comparing the x-axis values and y-axis values separately. in order to meet this condition, both and b must have the same sign. Either both must be positive or both must be negative.

-if both a and b are positive, then the two points will lie in the 2nd quadrant (negative x, positive y).
-if both a and b are negative, then the two points will lie in the 4th quadrant (positive x, negative y).

now we need to establish which quadrant does point (-x, y) lie in using the statements.

statement 1: xy > 0

either
x > 0, y > 0, so point (-x, y) will lie in the 2nd quadrant,

or
x < 0, y < 0, so point (-x, y) will lie in the 4th quadrant.

although the two earlier points also lie in one of these 2 quadrants, we still don’t know exactly which one. Hence insufficient.

statement 2: ax > 0

either
a > 0, x > 0, so points (-a, b) and (-b, a) will lie in the 2nd quadrant, but we have no info on y,

or
a < 0, x < 0, so points (-a, b) and (-b, a) will lie in the 4th quadrant, but we have no info on y.

in either case, we don’t know anything about y, hence cannot conclude anything. insufficient.

both statements together:

if x>0, y>0, then from S2 a>0 and hence b>0. this means all points are in the 2nd quadrant.
if x<0, y<0, then from S2 a<0 and hence b<0. this means all points are in the 4th quadrant.

in both cases we have (-x, y), (-a, b) and (-b, a) within the same quadrant. this answers the question stem as a definite YES. Sufficient.

Choose C.

-BM-
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