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Does x + y = 0?

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Does x + y = 0?  [#permalink]

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New post 11 Dec 2014, 06:05
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Project DS Butler: Day 60: Data Sufficiency (DS120)


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Tough and Tricky questions: Algebra.



Does x + y = 0?

(1) xy<0
(2) x^2 = y^2

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Re: Does x + y = 0?  [#permalink]

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New post 11 Dec 2014, 09:06
1
statement 1: insufficient
this only tells us that either x or y is positive and the other is negative

statement 2: insufficient
if x=2 and y=2, then 4=4 and going back to the original question, 2+2 does not equal 0. but if x=-2 and y=2, then we get a yes and it also fits statement 1.

statement 1+2: sufficient
knowing that one is positive and one is negative from statement 1, the only way statement 2 can be true is if the numbers are the same but have opposite symbols positive/negative. since that is the case, x+y will always be 0.

Answer C!
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Re: Does x + y = 0?  [#permalink]

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New post 11 Dec 2014, 23:46
1
Answer c.
statement 1: x, y are of opposite sign.
statement 2: |x| and |y| are equal
1 & 2: x= -y
x+y=0
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Re: Does x + y = 0?  [#permalink]

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New post 12 Dec 2014, 07:08
1) xy<0, it means that x and y has different signs, but not necessarily equal. NS
2) (x-y)(x+y)=0
either x=y or x=-y, as we are not certain that x+y=0, then it's NS
BOTH
as xy<0, the signs of x and y are different, then x=-y, or x+y=0 Suf
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Re: Does x + y = 0?  [#permalink]

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Re: Does x + y = 0?  [#permalink]

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New post 04 Jan 2019, 08:37
Does x + y = 0? (OR is -x=y OR x=-y)

(1) xy<0
(2) x^2 = y^2

So
1: xy<0 so either x OR y <0 Not sufficient

2. x^2 =y^2 . |x| = |y|. But we dont know anything about the sign of either x OR y. Not sufficient

Both together
xy<0 and x^2 =y^2 : So either one of them is negative, and they have same aboslute value. Suffient to answer the question x+y=0
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Re: Does x + y = 0?  [#permalink]

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New post 04 Jan 2019, 10:17
Statement 1
xy < 0
x can be +ive or -ive, or can be = to y
y can be +ive or -ive, or can be = to x

With those cases in mind, inline are some samples
x=1, y=-1, will answer the question as Yes
x=2, y =-1, will answer the question as No

Statement 2
x can be +ive or -ive
y can be +ive or -ive

With those cases in mind, inline are some samples
x=1, y=-1, will answer the question as Yes
x=1, y =1, will answer the question as No

Combine both the statements and you will always get a Yes
If x is +ive, y has to be -ive
If y is +ive, x has to be -ive.

With those cases in mind, inline are some samples
x=1, y=-1, will answer the question as Yes
x=-1, y =1, will answer the question as Yes

Correct answer C.
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Re: Does x + y = 0? &nbs [#permalink] 04 Jan 2019, 10:17
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