Positives/Negatives

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nycknicks11: Senior | Next Rank: 100 Posts; Posts: 56; Joined: Tue Apr 12, 2011 9:07 amFollowed by: 1 member; Thanked: 5 times

by nycknicks11 » Tue Jun 07, 2011 8:25 pm

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

Can someone explain why I am wrong here?

Problem: from the problem given, we know that a,b have the same sign but we don't know what it is. ++ or --.

1) xy>0 means both are the same sign this means that (-x,y) must have opposite signs. Thus this is sufficient to conclude that (-x,y) IS NOT in the same quadrant. Sufficient.

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Source: Beat The GMAT — Data Sufficiency | Tue Jun 07, 2011 8:25 pm

phanideepak: Senior | Next Rank: 100 Posts; Posts: 77; Joined: Wed Apr 27, 2011 6:13 amFollowed by: 2 members; Location: Hyderabad; Thanked: 10 times; GMAT Score:730

by phanideepak » Tue Jun 07, 2011 8:35 pm

Are you sure that you are wrong?

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cans: Legendary Member; Posts: 1309; Joined: Mon Apr 04, 2011 5:34 amFollowed by: 123 members; Location: India; Thanked: 310 times; GMAT Score:750

by cans » Tue Jun 07, 2011 8:42 pm

A(-a,b) B(-b,a) lie in same quadrant and thus it means a and b are of same sign. they can be ++ or --
thus -a,b are of opposite signs and also -b,a are of opposite signs.
it means A,B are either in 2nd quadrant or in 4th quadrant
to find if C(-x,y) lies in same quadrant as A,B
a)xy>0
it means x,y are of same sign. thus -x,y are of opposite sign
Thus C lies either in 2nd or 4th quadrant
But we have possibility that A,B lie in 2nd while C lies in 4th
Or another possibility that A,B,C lie in 2nd. Thus insufficient
b)ax>0
it means a,x are of same sign. let a>0, A,B lie in 2nd quadrant
Also then x>0, or -x<0 and no information on C, thus C can lie either in 2nd quadrant or in 3rd quadrant
Insufficient
both together) C either in 2nd or in 4th. but x,y of same sign, thus y>0 and thus C in 2nd quadrant. Also A,B in 2nd only
Sufficient
IMO C

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Cans!!

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phanideepak: Senior | Next Rank: 100 Posts; Posts: 77; Joined: Wed Apr 27, 2011 6:13 amFollowed by: 2 members; Location: Hyderabad; Thanked: 10 times; GMAT Score:730

by phanideepak » Tue Jun 07, 2011 8:47 pm

@Cans Awesome explanation. Looks like i have to improve my comprehension of the question

I was assuming the question to be is(-x,y)in the same quadrant as (-y,x). Thanks for the expln!!

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