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# What is the value of x^2 + y^2 ?

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What is the value of x^2 + y^2 ? [#permalink]

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23 Nov 2012, 07:29
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Difficulty:

45% (medium)

Question Stats:

56% (02:11) correct 44% (01:13) wrong based on 150 sessions

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What is the value of x^2 + y^2 ?

(1) x^2 + y^2 = 2xy+1
(2) x^2 + y^2 = 4 - 2xy

I have an official answer but I was thinking that it is wrong.
This is a question from GMATHacks Algebra Challange set.

OA SOLUTION

[Reveal] Spoiler:
77. E
Explanation: This question rigorously tests your familiarity with common
binomials. The only way to do anything with statement (1) is to subtract 2xy
from both sides:
x^2 - 2xy + y^2 = 1
(x - y)(x - y) = 1
x - y = 1 or x - y = -1
There are an innite number of possible solutions for x and y, so theres no
way to determine a specic value for x^2 + y^2.
Statement (2) is insufficient for similar reasons:
x^2 + y^2 = 4 - 2xy
x^2 + 2xy + y^2 = 4
(x + y)(x + y) = 4
x + y = 2 or x + y = -2
Again, there are an innite number of possibilities for x and y.
Taken together, you still dont have enough information. If you had exactly
two equations (such as x-y = 1 and x+y = 2), you could solve, but you have
two diferent pairs of possible equations, which is insufficient to find the values
of the variables. Choice (E) is correct.
[Reveal] Spoiler: OA

Last edited by Bunuel on 24 Nov 2014, 01:44, edited 6 times in total.
Edited the OA.
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Re: What is the value of x^2 + y^2 ? [#permalink]

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23 Nov 2012, 07:41
3
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Expert's post
What is the value of x^2+y^2 ?

(1) x^2+y^2=2xy+1 --> $$x^2-2xy+y^2=1$$ --> $$(x-y)^2=1$$. If $$x=1$$ and $$y=0$$, then $$x^2+y^2=1$$ but if $$x=2$$ and $$y=1$$, then $$x^2+y^2=5$$. Not sufficient.

(2) x^2+y^2=4-2xy --> $$x^2+2xy+y^2=4$$ --> $$(x+y)^2=4$$. If $$x=2$$ and $$y=0$$, then $$x^2+y^2=4$$ but if $$x=1$$ and $$y=1$$, then $$x^2+y^2=2$$. Not sufficient.

(1)+(2) Sum the equations: $$2(x^2+y^2)=5$$ --> $$x^2+y^2=2.5$$. Sufficient.

Answer: C. OA must be wrong.

Hope it's clear.
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Re: What is the value of x^2 + y^2 ? [#permalink]

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23 Nov 2012, 07:46
Bunuel wrote:
What is the value of x^2+y^2 ?

(1) x^2+y^2=2xy+1 --> $$x^2-2xy+y^2=1$$ --> $$(x-y)^2=1$$. If $$x=1$$ and $$y=0$$, then $$x^2+y^2=1$$ but if $$x=2$$ and $$y=1$$, then $$x^2+y^2=5$$. Not sufficient.

(2) x^2+y^2=4-2xy --> $$x^2+2xy+y^2=4$$ --> $$(x+y)^2=4$$. If $$x=2$$ and $$y=0$$, then $$x^2+y^2=4$$ but if $$x=1$$ and $$y=1$$, then $$x^2+y^2=2$$. Not sufficient.

(1)+(2) Sum the equations: $$2(x^2+y^2)=5$$ --> $$x^2+y^2=2.5$$. Sufficient.

Answer: C. OA must be wrong.

Hope it's clear.

Thanks Bunuel!!! (I feel like a VIP having you respond to one of my questions!) haha

"C" is what I thought! I got worried and couldn't see what I was doing wrong even after reading the OA description.
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Re: What is the value of x^2 + y^2 ? [#permalink]

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23 Nov 2012, 07:48
maxLRok wrote:
Bunuel wrote:
What is the value of x^2+y^2 ?

(1) x^2+y^2=2xy+1 --> $$x^2-2xy+y^2=1$$ --> $$(x-y)^2=1$$. If $$x=1$$ and $$y=0$$, then $$x^2+y^2=1$$ but if $$x=2$$ and $$y=1$$, then $$x^2+y^2=5$$. Not sufficient.

(2) x^2+y^2=4-2xy --> $$x^2+2xy+y^2=4$$ --> $$(x+y)^2=4$$. If $$x=2$$ and $$y=0$$, then $$x^2+y^2=4$$ but if $$x=1$$ and $$y=1$$, then $$x^2+y^2=2$$. Not sufficient.

(1)+(2) Sum the equations: $$2(x^2+y^2)=5$$ --> $$x^2+y^2=2.5$$. Sufficient.

Answer: C. OA must be wrong.

Hope it's clear.

Thanks Bunuel!!! (I feel like a VIP having you respond to one of my questions!) haha

"C" is what I thought! I got worried and couldn't see what I was doing wrong even after reading the OA description.

Can you please post their solution?
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Re: What is the value of x^2 + y^2 ? [#permalink]

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23 Nov 2012, 08:50
Bunuel wrote:
maxLRok wrote:
Bunuel wrote:
What is the value of x^2+y^2 ?

(1) x^2+y^2=2xy+1 --> $$x^2-2xy+y^2=1$$ --> $$(x-y)^2=1$$. If $$x=1$$ and $$y=0$$, then $$x^2+y^2=1$$ but if $$x=2$$ and $$y=1$$, then $$x^2+y^2=5$$. Not sufficient.

(2) x^2+y^2=4-2xy --> $$x^2+2xy+y^2=4$$ --> $$(x+y)^2=4$$. If $$x=2$$ and $$y=0$$, then $$x^2+y^2=4$$ but if $$x=1$$ and $$y=1$$, then $$x^2+y^2=2$$. Not sufficient.

(1)+(2) Sum the equations: $$2(x^2+y^2)=5$$ --> $$x^2+y^2=2.5$$. Sufficient.

Answer: C. OA must be wrong.

Hope it's clear.

Thanks Bunuel!!! (I feel like a VIP having you respond to one of my questions!) haha

"C" is what I thought! I got worried and couldn't see what I was doing wrong even after reading the OA description.

Can you please post their solution?

Just put it in the main post
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Re: What is the value of x^2 + y^2 ? [#permalink]

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23 Nov 2012, 08:56
maxLRok wrote:
Bunuel wrote:
maxLRok wrote:

Thanks Bunuel!!! (I feel like a VIP having you respond to one of my questions!) haha

"C" is what I thought! I got worried and couldn't see what I was doing wrong even after reading the OA description.

Can you please post their solution?

Just put it in the main post

Their solution is not correct.

Four pairs of (x,y) satisfy (x-y)^2=1 and (x+y)^2=4: (-3/2, -1/2), (-1/2, -3/2), (3/2, 1/2) and (1/2, 3/2). For each pair x^2+y^2=10/2.
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Re: What is the value of x^2 + y^2 ? [#permalink]

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06 Jun 2014, 07:02
Hello Everyone,

This DS problem is from the Jeffrey Sackmann Challenge Algebra set, Quadratics. Not sure about the difficulty level.

What is the value of x^2 + y^2 ?
1) x^2 + y^2 = 2xy + 1
2) x^2 + y^2 = 4 - 2xy

The answer is (E) But I'm not sure why we didn't just use the two statements together > add them > 2xy cancels out and the answer would be: x^2 + y^2 = 5/2 ?

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Re: What is the value of x^2 + y^2 ? [#permalink]

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06 Jun 2014, 07:08
1
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Expert's post
Mfz83 wrote:
Hello Everyone,

This DS problem is from the Jeffrey Sackmann Challenge Algebra set, Quadratics. Not sure about the difficulty level.

What is the value of x^2 + y^2 ?
1) x^2 + y^2 = 2xy + 1
2) x^2 + y^2 = 4 - 2xy

The answer is (E) But I'm not sure why we didn't just use the two statements together > add them > 2xy cancels out and the answer would be: x^2 + y^2 = 5/2 ?

Merging similar topics. please refer to the discussion above.

You are right the OA must be C, instead of E.

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Re: What is the value of x^2 + y^2 ? [#permalink]

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11 Jun 2014, 04:41
What is the value of $$x^2$$ +$$y^2$$?

(1)$$x^2$$ +$$y^2$$ = $$2xy + 1$$

(1) $$x^2 +y^2 = 4 - 2xy$$

Last edited by Gnpth on 11 Jun 2014, 04:58, edited 1 time in total.
Updated the OA
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Re: What is the value of x^2 + y^2 ? [#permalink]

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11 Jun 2014, 05:01
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irda wrote:
What is the value of $$x^2$$ +$$y^2$$?

(1)$$x^2$$ +$$y^2$$ = $$2xy + 1$$

(1) $$x^2 +y^2 = 4 - 2xy$$

The question is already discussed here. Also the OA is wrong.

Before posting read the rules for posting.
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Re: What is the value of x^2 + y^2 ? [#permalink]

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Re: What is the value of x^2 + y^2 ? [#permalink]

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06 Jul 2016, 06:39
maxLRok wrote:
What is the value of x^2 + y^2 ?

(1) x^2 + y^2 = 2xy+1
(2) x^2 + y^2 = 4 - 2xy

Statement 1 and 2 both do not give us the value of xy and hence are not sufficient to arrive at the answer

2(x^2 + y^2)= 5

x^2 + y^2= 2.5

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Re: What is the value of x^2 + y^2 ?   [#permalink] 06 Jul 2016, 06:39
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