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What is the value of a + b? (1) a^2 + b^2 = 13 (2) 2b = 12/a

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Manager
Joined: 16 Oct 2008
Posts: 54
What is the value of a + b? (1) a^2 + b^2 = 13 (2) 2b = 12/a [#permalink]

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17 Nov 2008, 02:02
What is the value of a + b?
(1) a^2 + b^2 = 13
(2) 2b = 12/a

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Director
Joined: 14 Aug 2007
Posts: 697

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17 Nov 2008, 02:09
Natasha0123 wrote:
What is the value of a + b?
(1) a^2 + b^2 = 13
(2) 2b = 12/a

1,2 are individually insuff .

together we have

a^2+2ab+b^2= 13 + 2ab
(a+b)^2 = 15
a+b = sqrt(15)

C
Manager
Joined: 30 Sep 2008
Posts: 111

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17 Nov 2008, 02:13
(1) & (2)

a^2 + b^2 = (a+b)^2 - 2ab = 13

=> (a+b)^2 = 13 + 2ab = 13 + 2*6 = 25

=> a + b = 5 or a + b = -5

E
Director
Joined: 14 Aug 2007
Posts: 697

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17 Nov 2008, 02:32
Natasha0123 wrote:
alpha_plus_gamma wrote:
Natasha0123 wrote:
What is the value of a + b?
(1) a^2 + b^2 = 13
(2) 2b = 12/a

1,2 are individually insuff .

together we have

a^2+2ab+b^2= 13 + 2ab
(a+b)^2 = 15
a+b = sqrt(15)

C

why 13 + 2ab = 15?

Oh sorry that was typo, i meant 13+12=25
Intern
Joined: 25 Mar 2008
Posts: 43

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20 Nov 2008, 23:32
whats the OA for this ?

is it C ?

GMAT does not support negative square root.

thanks
Manager
Joined: 02 Nov 2008
Posts: 56

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21 Nov 2008, 09:39
I would go with E. Don't know if a+b = -5 or +5
Retired Moderator
Joined: 05 Jul 2006
Posts: 1734

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22 Nov 2008, 08:01
Natasha0123 wrote:
What is the value of a + b?
(1) a^2 + b^2 = 13
(2) 2b = 12/a

EACH ALONE IS INSUFF

both

a^2+b^2+2ab = 25 ie: (a+b)^2 = 25

a+b can either 5 or -5............E
Manager
Joined: 16 Oct 2008
Posts: 54

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26 Nov 2008, 04:14
I also pick E but OA is C

c. Remember that (a + b)2 = a2 + 2ab + b2. From statement (1), we know that a2 + b2 = 13. By crossmultiplying
in statement (2), we get 2ab = 12. Since we know the values of a2 + b2 and 2ab, and
(a + b)2 = a2 + 2ab + b2, we can now take the square root of the sum to find the value of a + b.

source : Gmat Exam Success book
VP
Joined: 05 Jul 2008
Posts: 1340

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26 Nov 2008, 15:06
If a book says OA is C, The book you are working on has serious issues.

I got E because a+b can be 5 or -5
Manager
Joined: 27 May 2008
Posts: 193

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27 Nov 2008, 23:42
you are absolutely correct icandy
VP
Joined: 17 Jun 2008
Posts: 1479

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28 Nov 2008, 04:45
yes, OA must be E (unless the question stem says that a and b are positive, or negative, etc.).

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

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Re: No. prop Q   [#permalink] 28 Nov 2008, 04:45
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What is the value of a + b? (1) a^2 + b^2 = 13 (2) 2b = 12/a

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