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# What is the value of ab?

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Intern
Joined: 31 Oct 2011
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Schools: ESSEC '15 (A)
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What is the value of ab? [#permalink]

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09 Dec 2012, 10:45
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Difficulty:

35% (medium)

Question Stats:

64% (01:50) correct 36% (00:59) wrong based on 134 sessions

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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, 11:06, edited 1 time in total.
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Re: What is the value of ab? [#permalink]

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09 Dec 2012, 11:01
Expert's post
1) clearly insuff

2) also manipulate it is insuff

1) + 2) $$a= 1$$ $$b = 0$$

$$a*b=0$$

C suff for me
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Re: What is the value of ab? [#permalink]

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09 Dec 2012, 11:09
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Expert's post
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.

P.S. Please provide OA's when posting questions.
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Re: What is the value of ab? [#permalink]

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12 Jan 2014, 07:38
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Re: What is the value of ab? [#permalink]

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01 Apr 2014, 23:45
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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02 Apr 2014, 00:09
Expert's post
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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Re: What is the value of ab?   [#permalink] 02 Apr 2014, 00:09
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