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Senior Manager
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If ab>0, does (a+2)(b+2) = 4? (1) ab = -2(a+b) (2) a=b [#permalink]
 10 Sep 2005, 15:06
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If ab>0, does (a+2)(b+2) = 4?
(1) ab = -2(a+b)
(2) a=b
(Never mind, I realize what I did wrong...)
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Senior Manager
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Yeah, OA is A.
I of course went through a whole elaborate solve method for B in which I assumed the solution while trying to determine it. Bad idea. 
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Current Student
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why is it A?
Always attempt to give an explanation if you can....just so people can learn from it...
here is how I was thinking....
If ab>0, does (a+2)(b+2) = 4?
(1) ab = -2(a+b)
(2) a=b
so I re-arrange the stem (a+b)(4)=4...
so we need to know is if (a+b)=1
(1) says ab=-2(a+b)
then (we know ab>0) -ab/2=(a+b) or -1/2=[(a+b)/ab]...clearly (a+b) is not equal to 1...so (1) is sufficient...
(2) a=b
we know that a=b, well we really need to know if a=1 or 1/2 or b=1 or 1/2...this doesnt say much about a or b....so Insufficient...
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fresinha12 wrote:
so I re-arrange the stem (a+b)(4)=4...
how did you re-arrange (a+2)(b+2)=4 to (a+b)(4)=4?
Am I missing something?
I choose (D) (though I see the OA mentioned as A)
stem asks whether (a+2)(b+2)=4?
this can be re-arranged to ab+2a+2b=0
statement I: ab=-2(a+b), which can be rewritten to ab+2a+2b=0, thus sufficient.
statement II: a=b. is (a+2)(a+2)=4? (a+2)^2=4, or a+2=+2 or -2, therefore a = -4, since if a+2=+2, a would equal zero, and ab would not be > 0. since a = b, a = b = -4. therefore (-4+2)(-4+2)=4. Sufficient.
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Chets, can you explain how we can just solve the equation (a+2)(b+2) = 4 when the stem says we are not sure if (a+2)(b+2) is indeed 4.
From 2) all you really get is (a+2)(a+2) which is (a+2)^2.. .which does not mean that the eqn is necessarily = 4. a can be anything.... 1,2, -5, 3/4, Depending on what value of a you select ... (a+2)^2 can have any value. So insufficient. All it means is that for the condition to be true, we need a=2 and a=b does not tell you that.
_________________
-Vikram
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vikramm wrote: Chets, can you explain how we can just solve the equation (a+2)(b+2) = 4 when the stem says we are not sure if (a+2)(b+2) is indeed 4.
From 2) all you really get is (a+2)(a+2) which is (a+2)^2.. .which does not mean that the eqn is necessarily = 4. a can be anything.... 1,2, -5, 3/4, Depending on what value of a you select ... (a+2)^2 can have any value. So insufficient. All it means is that for the condition to be true, we need a=2 and a=b does not tell you that.
Good call, statement II doesn't suffice. So I guess A it is!
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It is A,
Probably a question asked at the end of a GMAT exam. Possibility that it is an experimental question...
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Re: DS - never mind, I realize what I did wrong [#permalink]
03 Oct 2005, 03:01
coffeeloverfreak wrote: If ab>0, does (a+2)(b+2) = 4?
(1) ab = -2(a+b)
(2) a=b
(Never mind, I realize what I did wrong...)
My answer is A. Here is the explanation.
(a+2) (b+2) = ab + 2a + 2b + 4
=> ab + 2(a+b) + 4
From statement 1 we have ab = -2(a+b), substituting in above equn,
-2(a+b) + 2(a+b) + 4
= 4 , So A is sufficient to answer the solution.
From statement 2, we get a = b applying this in original equn we get
a^2+4+4a , here the value will differ for every value of A. Hence B is not sufficient.
Thanks
Sreeni
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Re: DS - never mind, I realize what I did wrong
[#permalink]
03 Oct 2005, 03:01
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