What is the value of ab? : GMAT Data Sufficiency (DS)
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What is the value of ab?

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What is the value of ab? [#permalink]

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What is the value of ab?

(1) a = b + 1
(2) a^2 = b + 1
[Reveal] Spoiler: OA

Last edited by Bunuel on 09 Dec 2012, 10:06, edited 1 time in total.
OA added.
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Re: What is the value of ab? [#permalink]

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New post 09 Dec 2012, 10:01
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Re: What is the value of ab? [#permalink]

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Maxswe wrote:
What is the value of ab?

(1) a = b + 1
(2) a^2 = b + 1


Each statement alone is clearly insufficient. When taken together we have that a=a^2, which implies that a=0 or a=1.
If a=0 --> ab=0;
If a=1 --> b=0 --> ab=0.

Sufficient.

Answer: C.

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Re: What is the value of ab? [#permalink]

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Re: What is the value of ab? [#permalink]

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New post 01 Apr 2014, 22:45
I had a question on this question. I thought the answer should be B.

a^2=b+1...then a^2-1=b
a^2-1 can be factored and would equal (a+1)(a-1)
then (a+1)(a-1)=b
in this case a = -1 or 1. When plugging both -1 or 1 in b is always zero.

therefore ab will always equal 0.

Is it wrong to factor out the equation in the second statement?
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Re: What is the value of ab? [#permalink]

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New post 01 Apr 2014, 23:09
amy888 wrote:
I had a question on this question. I thought the answer should be B.

a^2=b+1...then a^2-1=b
a^2-1 can be factored and would equal (a+1)(a-1)
then (a+1)(a-1)=b
in this case a = -1 or 1. When plugging both -1 or 1 in b is always zero.

therefore ab will always equal 0.

Is it wrong to factor out the equation in the second statement?


Factoring is correct but the derivation is not.

(a+1)(a-1)=b does not mean that a=-1 or a=1, it would be so if we knew that b=0. I mean if it were (a+1)(a-1)=0, then yes, for the product of two multiples, a+1 and a-1, to be 0 either of them must be zero, hence a=-1 or a=1.
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: What is the value of ab?   [#permalink] 01 Apr 2014, 23:09
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