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# What is the value of ab?

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Intern
Joined: 31 Oct 2011
Posts: 19
Schools: ESSEC '15 (A)
GMAT 1: 650 Q45 V35
What is the value of ab? [#permalink]

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Updated on: 09 Dec 2012, 11:06
1
00:00

Difficulty:

35% (medium)

Question Stats:

67% (00:53) correct 33% (00:58) wrong based on 203 sessions

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What is the value of ab?

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

Originally posted by Maxswe on 09 Dec 2012, 10:45.
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
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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Joined: 02 Sep 2009
Posts: 47110
Re: What is the value of ab? [#permalink]

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09 Dec 2012, 11:09
1
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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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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Joined: 02 Sep 2009
Posts: 47110
Re: What is the value of ab? [#permalink]

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02 Apr 2014, 00:09
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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What is the value of ab? [#permalink]

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17 Aug 2017, 19:53
Gnpth wrote:
What is the value of ab?

1. $$a= b+1$$

2. $$a^2= b+1$$

Each statement individually can be seen is not sufficient.
Let's see what so we get when combined

a=b+1 and $$a^2=b+1..... So ...a=a^2.....a^2-a=0....a(a-1)=0$$
Thus a is 0 or 1...

If a =0, a*b=0 irrespective of value of b otherwise b=a-1=-1
If a=1, b=a-1=1-1=0... So a*b=1*0=0

So in each case ANSWER is 0
Sufficient

C
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1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

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What is the value of ab?   [#permalink] 17 Aug 2017, 19:53
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