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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.

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. _________________

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!

Re: DS - never mind, I realize what I did wrong [#permalink]
03 Oct 2005, 02:01

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

gmatclubot

Re: DS - never mind, I realize what I did wrong
[#permalink]
03 Oct 2005, 02:01