Points that like in the same xy-coordinate plane as (a, b)

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Hello,

I had a question here:

Which of the following points could lie in the same quadrant of the xy-coordinate plane as the point (a, b), where ab ≠ 0 ?

A) (-b, -a)

B) (-a, -b)

C) (b, -a)

D) (a, -b)

E) (-b, a)

OA: A


I picked a = 2, b = 3 and then got the following values for the answer choices:

(-b, -a ) = (-3, -2)
(-a, -b ) = (-2, -3)
(b, -a ) = (3, -2)
(a, -b ) = (2, -3)
(-b, a ) = (-3, 2)

I am thinking that for the correct answer, one of the above choices should lie in the same quadrant as a = 2 and b = 3 i.e. Quadrant 1. None of the above choices meet that condition.

I then picked a = -2 and b = 3 and they lie in the Quadrant 2. Now we have:

(-b, -a) = (-3, 2)
(-a, -b) = (2, -3)
(b, -a) = (3, 2)
(a, -b) = (-2, -3)
(-b, a) = (-3, -2)

In the above choices, (-b, -a) = (-3, 2) also lies in Quadrant 2. Hence, this would be a correct value. I was wondering if there is a way we can directly test the answer choices for a point in Quadrant 2 without testing the answer choices for a point in Quadrant 1? Thanks a lot for your help.

Best Regards,
Sri
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by theCodeToGMAT » Wed May 14, 2014 11:24 pm
let a = 4 or b = -3 ==> Quad 4th

A) (-b, -a)
==> (3,-4) = 4th

B) (-a, -b)
==> (-4, 3) = 2rd

C) (b, -a)
==> (-3,-4) = 3rd

D) (a, -b)
==> (4,3) = 1st

E) (-b, a)
==> (3, 4) = 1st

[spoiler]Only {A} [/spoiler]
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by [email protected] » Fri May 16, 2014 11:45 am
Hi Sri,

Your idea to TEST VALUES was a good one. The way that the question is worded points out a specific "quirk" to this question:

"Which of the following points COULD lie in the same quadrant...."

This ultimately means that 4 of the points COULD NOT like in the same quadrant while just 1 COULD SOMETIMES (but not necessarily every time) lie in the same quadrant. This means that the first set of values that you try might not be enough to answer the question, so you need to be prepared to do a little more work than normal to get this one correct.

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by Matt@VeritasPrep » Fri May 16, 2014 11:52 am
Yeah, you can always test whichever quadrants you like: you aren't obligated to start with the first quadrant.

One thing to notice here, though: (a, b) and (-b, -a) can't both lie in Quadrant I, as there's no way to have a, b, -b, and -a all be positive: two of those (though we don't know which two) MUST be negative. Quadrant IV will have the same issue (two of a, b, -b, and -a MUST be positive), so you only have to test Quadrants II and III.