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What is the value of x + y?

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

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New post 23 Jul 2015, 02:55
1
5
00:00
A
B
C
D
E

Difficulty:

  45% (medium)

Question Stats:

60% (01:14) correct 40% (00:58) wrong based on 86 sessions

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

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New post 23 Jul 2015, 03:11
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Bunuel wrote:
What is the value of x + y?

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

Kudos for a correct solution.


Question : x+y = ?

Statement 1: x^2 + y^2 = 5
(x+y)^2 = x^2 + y^2 + 2xy
but due to lack of information about 2xy, (x+y) can't be calculated. Hence,
NOT SUFFICIENT

Statement 2: xy = 2
(x+y)^2 = x^2 + y^2 + 2xy
but due to lack of information aboutx^2 + y^2, (x+y) can't be calculated. Hence,
NOT SUFFICIENT

Combining the two statements:
(x+y)^2 = x^2 + y^2 + 2xy = 5+2*2 = 9
i.e. (x+y) = +3
Two possible values of (x+y), hence
NOT SUFFICIENT

Answer: Option E
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Re: What is the value of x + y?  [#permalink]

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New post 23 Jul 2015, 04:37
Bunuel wrote:
What is the value of x + y?

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

Kudos for a correct solution.


x+y=?

Per statement 1, x^2+y^2=5 ---> x = 1 , y = 2, -2 ---> 1+2 or 1-2 . Thus this statement is not sufficient.

Per statement 2, xy = 2--->x =1, y =2 or x =0.5, y =4 . Thus we get 1+2 or 0.5+4 . Thus this statement is not sufficient.

Combining, we get,

x^2+y^2=5 ---> (x+y)^2 - 2xy = 5 ---> (x+y)^2 = 5+2*2 = 9 ---> (x+y) 3 or -3. Thus we do not get a unique value for x+y ---> E is the correct answer.
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Re: What is the value of x + y?  [#permalink]

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New post 23 Jul 2015, 07:05
Bunuel wrote:
What is the value of x + y?

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

Kudos for a correct solution.


Statement 1:x^2 + y^2 = 5. Insufficient
Statement 2:xy=2. Insufficient
Using both statements, (x+y)^2-2xy=5
(x+y)^2-4=5
(x+y)^2=9
x+y=+3 or -3
Answer E
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Re: What is the value of x + y?  [#permalink]

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New post 26 Jul 2015, 12:44
Bunuel wrote:
What is the value of x + y?

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

Kudos for a correct solution.


800score Official Solution:

If we use statement (1) by itself, we can solve it for x. x² = 5 – y². This yields two possible solutions, x = -√(5 – y²) and x = √(5 – y²). The desired sum, x + y, can be y + √(5 – y²) and y – √(5 – y²). Therefore we do NOT have a definite value of the desired sum. Statement (1) by itself is NOT sufficient.

If we use statement (2) by itself, it implements that y can NOT be 0 and we can solve it for x. x = 2/y. The desired sum, x + y, is 2/y + y. We do NOT know the value of y. Therefore we do NOT have a definite value of the desired sum. Statement (2) by itself is NOT sufficient.

If we use the both statements together, let’s manipulate them a little. First, multiply the second one by 2 to get 2xy = 4. Then add the first one to it. x² + 2xy + y² = 5 + 4. In the left side we have a well known formula of the square of a sum.
(x + y)² = 9. It yields two solutions, x + y = 3 and x + y = -3. If you plug in x = 2 and y = 1 or x = -2 and y = -1, then you'll see that the both options are possible. Statements (1) and (2) taken together are NOT sufficient to answer the question.

The correct answer is E.
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Re: What is the value of x + y?  [#permalink]

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New post 21 Mar 2019, 08:38
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Re: What is the value of x + y?   [#permalink] 21 Mar 2019, 08:38
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