GMAT PREP : Co-ordinate/Number Theory

This topic has expert replies
User avatar
Master | Next Rank: 500 Posts
Posts: 246
Joined: Mon May 19, 2008 7:34 am
Location: Texaco Gas Station
Thanked: 7 times
My reasoning was as a and b should have same sign (+/-) x should have opposite sign of a...where is it wrong?
Attachments
gmatprep06190802.jpg
Cubicle Bound Misfit

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2621
Joined: Mon Jun 02, 2008 3:17 am
Location: Montreal
Thanked: 1090 times
Followed by:355 members
GMAT Score:780

by Ian Stewart » Thu Jun 19, 2008 7:03 pm
If (-x, y) is in the same quadrant as (-a, b), then x and a have the same sign, and y and b have the same sign.

You're certainly right that a and b have the same sign; that information is in the question itself. I wonder if you combined both statements accidentally when you chose your answer.

Statement 1 alone tells us that x and y have the same sign; that isn't enough unless we know some relationship between the signs of x and y and the signs of a and b.

Statement 2 alone tells us that a and x have the same sign. We still don't know about b and y.

Together, from 1, y and x have the same sign, and from 2, x and a have the same sign, and from the question, a and b have the same sign. So every letter has the same sign, and we can certainly answer the question.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com