Which of the following points could lie in the same quadrant

Question

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)

Bunuel · Accepted Answer

If a is positive and b is positive: (a, b) = (+, +), then (–b, –a) = (-, -). For example, if (a, b) = (1, 1), then (–b, –a) = (-1, -1). So, (a, b) and (–b, –a) are NOT in the same quadrant.

If a is positive and b is negative: (a, b) = (+, -), then (–b, –a) = (+, -). For example, if (a, b) = (1, -1), then (–b, –a) = (1, -1). So, (a, b) and (–b, –a) are in the same quadrant.

If a is negative and b is positive: (a, b) = (-, +), then (–b, –a) = (-, +). For example, if (a, b) = (-1, 1), then (–b, –a) = (-1, 1). So, (a, b) and (–b, –a) are in the same quadrant.

If a is negative and b is negative: (a, b) = (-, -), then (–b, –a) = (+, +). For example, if (a, b) = (-1, -1), then (–b, –a) = (1, 1). So, (a, b) and (–b, –a) are NOT in the same quadrant.

Hope it's clear.

Bunuel · Answer

First of all discard all options which have -a as the x-coordinate and -b as y-coordinate: eliminate B, and D.

(a, b) = (+, +), (+, -), (-, +), (-, -). Substitute in each option, to see which will match.

A. (–b, –a) --> (-, -),  (+, -) ,  (-, +) , (+, +). Match. No need to continue.

Answer: A.

avigutman · Answer

Video solution from Quant Reasoning starts at 0:25 Subscribe for more: https://www.youtube.com/QuantReasoning? ... irmation=1 https://www.youtube.com/watch?v=2iQ7YkK2oYo

hamzakb · Answer

I don;t understand how you did this . Could you elaborate by example?

e.g if (a,b) = (3,4), then how can (-4,-3) lie in the same quadrant?

(3,4) is in the first quadrant and (-4,-3) is in the 3rd. 
Confusing

romeokillsu · Answer

First of all discard all options which have -a as the x-coordinate and -b as y-coordinate: eliminate B, and D.

(a, b) = (+, +), (+, -), (-, +), (-, -). Substitute in each option, to see which will match.

A. (–b, –a) --> (-, -),  (+, -) ,  (-, +) , (+, +). Match. No need to continue.

Answer: A.[/quote

Please explain by example.. Thank you

robinpallickal · Answer

Great explanation Bunuel. Thanks a lot!!!!

Swaroopdev · Answer

Bunuel ,

Could you please explain on what basis you eliminated B&D ? Also could you share some background information on how you got the answer as A ? I.e. Those points matched only for two quadrants, how diid you conclude this as the answer ?

Basically I'm confused over the method you have used and on what basis you have concluded the answer too.

I got the correct answer as A, but i used just (2, -1) randomly as (a, b) and checked for all the options and only option A coordinates were on same quadrant as (2, -1). Please correct me if I'm wrong.

Thanks.

Bunuel · Answer

Have you read this: which-of-the-following-points-could-lie-in-the-same-quadrant-174711.html#p1388348

Swaroopdev · Answer

Bunuel , yes i have gone through the whole thing and only then posted those questions. What i don't understand in that is how you came to conclusion when only two points matched and other two didn't. And the rest i have put up in my previous post.

Thanks.

Bunuel · Answer

Given point: (a, b).

If a is positive, then -a is negative, and vise-versa. So, point (-a, ...) cannot be in the same quadrant as (a, b)
If b is positive, then -b is negative, and vise-versa. So, point (..., -b) cannot be in the same quadrant as (a, b)

I don't understand what you mean by "only two points matched and other two didn't".

ENGRTOMBA2018 · Answer

I believe you are confusing a must be true question with this question (a "could be true" ) question. In a could be true question, as soon as match 1 option , no need to check for ALL possible cases. For a must be true question, you need to make sure that ALL possible values are satisfied for an option to be the correct one.

Swaroopdev · Answer

Engr2012 ,  Bunuel  my confusion was not regarding checking other options such as B, C, D and E but with the below solution. Below two points are proved to be of same quadrant and two are not, is it okay only if two quadrant checks out fine or even if one quadrant is matched we can conclude the answer ?

If a is positive and b is positive: (a, b) = (+, +), then (–b, –a) = (-, -). For example, if (a, b) = (1, 1), then (–b, –a) = (-1, -1). So, (a, b) and (–b, –a) are NOT in the same quadrant.

If a is positive and b is negative: (a, b) = (+, -), then (–b, –a) = (-, -). For example, if (a, b) = (1, -1), then (–b, –a) = (1, -1). So, (a, b) and (–b, –a) are in the same quadrant.

If a is negative and b is positive: (a, b) = (-, +), then (–b, –a) = (-, +). For example, if (a, b) = (-1, 1), then (–b, –a) = (-1, 1). So, (a, b) and (–b, –a) are in the same quadrant.

If a is negative and b is negative: (a, b) = (-, -), then (–b, –a) = (+, +). For example, if (a, b) = (-1, -1), then (–b, –a) = (1, 1). So, (a, b) and (–b, –a) are NOT in the same quadrant.

ENGRTOMBA2018 · Answer

Good question. But Bunuel and I have mentioned, there are 2 things that you need to note for this question:

1. this is a could be true question, so even 1 correct/matching solution should tell you to stop.
2. Based on above 4 cases, you are able to get 1 value (-b,-a) to the point for the same quadrant. yes, for a question such as this one, you can conlcude once you get 1 matching option.

Bunuel provided these explanations in addition to his oginal solution post above. This post of his should not be treated as the complete solution.

Swaroopdev · Answer

Engr2012 and Bunuel , thanks to both of you, now my doubts are cleared.

Nunuboy1994 · Answer

Really, a fast way to solve question is just to consider a mixed pair of coordinates, for example (-3,2). If we have a pair of coordinates with the same values then none of these answers would apply.

Thus 
 "A"

EMPOWERgmatRichC · Answer

Hi All,

Co-ordinate Geometry (or "graphing", as most people call it) is a relatively rare category in the GMAT Quant section; you'll likely see just 1 of these questions on Test Day. The question is perfect for TESTing VALUES. Here's how:

We're asked which of the 5 answers COULD be in the same quadrant as (A,B), where neither A nor B equals 0. This makes me think that we'll have to consider more than one possibility, since there are 4 different quadrants on a graph.

Here are the examples that I would consider: 
(A,B) 
(1,2) - Quadrant 1 
(-1,2) - Quadrant 2 
(-1,-2) - Quadrant 3 
(1,-2) - Quadrant 4

You'll notice that each of the 5 answer choices changes the "sign" of at least one of the variables (and sometimes switches the variables around). If you start off in Quadrant 1, the only way to end up in that SAME Quadrant is if both the a and b are positive. That doesn't happen in ANY of the answer choices, so we need to look at a diffent Quadrant. I'm going to start with Quadrant 2:

Quadrant 2: 
(A,B) 
(-1,2)

So, if we plug A = -1 and B = 2 into the 5 answer choices, do any of them give us an answer that puts us in Quadrant 2? One of them DOES....

Final Answer:  A

GMAT assassins aren't born, they're made, 
Rich

BrentGMATPrepNow · Answer

Let's test some coordinates...

How about: a =  1  and b =  -2 .
This means the point (a, b) = ( 1 ,  -2 ), which is in QUADRANT IV

Now plug a =  1  and b =  -2  into each answer choice...

A) (-b, -a) =  -2 ), - 1 ] = (2, -1), which is also in QUADRANT IV 
PERFECT!

Answer: A

Cheers,
Brent

fskilnik · Answer

Just a quick observation that is (I hope) interesting in its own right:

If x*y and w*z are both different from zero, points (x,y) and (w,z) will be in the same quadrant if, and only if, 
x*w> 0 ( "x-coordinates" have the same sign ) and y*z>0 ( "y-coordinates" have the same sign ).

Conclusion:

(B) is refuted, because a and -a (x-coordinates in question stem and in this alternative choice) do not have the same signs (a is not zero) 
(C) is refuted, because if a and b (x-coordinates...) have the same signs , then b and -a (y-coordinates...) don´t have 
(D) is refuted, because b and -b (y-coordinates...) do not have the same signs (b is not zero) 
(E) is refuted, because if a and -b (x-coordinates...) have the same signs , then b and a (y-coordinates...) don´t have

Regards, 
fskilnik.

Izzyjolly · Answer

M(x_1, y_1)  and  N(x_2, y_2)  lie in the same quadrant, thus:  x_1 * y_1  and  x_2 * y_2  have the same sign.  (the opposite is not true) 
--> Quickly eliminate C, D, E.

Down to A & B.
B. the x-coordinate is -a. Point (-a,...) and Point (a,...) can't lie in the same quadrant. -> B is out.

A is the winner.

Which of the following points could lie in the same quadrant

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GMAT Problem Solving (PS) Questions

Which of the following points could lie in the same quadrant

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