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DS..Explaination needed

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Director
Joined: 16 May 2007
Posts: 548

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07 Jun 2007, 19:38
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Is A < 0 ?

1. -A = |-A|
2. A^2 = 9

Can someone please explain why the answer is C . I chose E.

Does the product of M and N equal to 1 ?
1. A*B*A = A
2. B*A*B = B

Why the answer is E. I chose D.
Manager
Joined: 28 Aug 2006
Posts: 160

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07 Jun 2007, 20:46
Is A < 0 ?

1. -A = |-A|
2. A^2 = 9

1)I think answer should be A

2) How is M & N related to A & B??Please make sure question is complete. thx
Director
Joined: 26 Feb 2006
Posts: 899

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07 Jun 2007, 21:55
trahul4 wrote:
Does the product of M and N equal to 1 ?
1. A*B*A = A
2. B*A*B = B

agree with E but i never saw such question in gmat.
Senior Manager
Joined: 11 May 2004
Posts: 328

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08 Jun 2007, 04:29
trahul4 wrote:
Is A < 0 ?

1. -A = |-A|
2. A^2 = 9

Can someone please explain why the answer is C . I chose E.

Does the product of M and N equal to 1 ?
1. A*B*A = A
2. B*A*B = B

Why the answer is E. I chose D.

From (1)

A either 0 or a negative number - Alone not sufficient

From (2)
A either -3 or 3 - Alone not sufficient

From both statement we can say A = 3

So, my choice E
Director
Joined: 30 Nov 2006
Posts: 591
Location: Kuwait

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08 Jun 2007, 07:41
1. -A =|-A|
A has to be negative
OR A could be zero, an integer that satisfies the condition above.

statement 1 is insufficient

2. A^2 = 9
-------------
Here either A is negative or positive.

statement 2 is insufficient

(1) and (2) together
-------------------------
A is either negative or zero from (1) and A is either negative or positive from (2) --> A is zero

Last edited by Mishari on 08 Jun 2007, 09:26, edited 1 time in total.
Director
Joined: 30 Nov 2006
Posts: 591
Location: Kuwait

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08 Jun 2007, 07:45
the second question .. please clarify .. A,B versus M and N ?
Director
Joined: 16 May 2007
Posts: 548

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08 Jun 2007, 07:59
Sorry for the typo mistake .. here it goes second question

Does the product of A and B equal to 1 ?
1. A*B*A = A
2. B*A*B = B
Manager
Joined: 18 Sep 2006
Posts: 58

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08 Jun 2007, 08:15
[quote="trahul4"]Sorry for the typo mistake .. here it goes second question

Does the product of A and B equal to 1 ?
1. A*B*A = A
2. B*A*B = B[/quote]\

Solution:
I would go for E;
Stmt1: A*B*A = A; this implies that B = 1 or B = 0 as (A^2 * B) = A ; but we don't know anything about A, so Insufficient

Stmt2: B*A*B = B; implies that A = 1 or A = 0 as (B^2 * A) = B; but we don't know anything about B from this statement, therefore insufficient

Stmt1 + Stmt2; Still insufficient because A*B could be 1 or 0
SVP
Joined: 01 May 2006
Posts: 1796

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08 Jun 2007, 08:27
Is A < 0 ?

1. -A = |-A|
2. A^2 = 9

---------------

(C) it is

From 1
-A = |-A|
=> -A >= 0 as an absolute value is always positive or nul
<=> A =< 0 >>> We remain with the case of A = 0

INSUFF.

From 2
A^2 = 9
implies that
o A = 3 (> 0)
or
o A = -3 (< 0)

INSUFF.

Combining (1) & (2)
From 1, we know that A =< 0. That forces with what we know from 2 (A= +/- 3) A to be equal to -3.

SUFF.
Director
Joined: 30 Nov 2006
Posts: 591
Location: Kuwait

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08 Jun 2007, 09:25
To find whether AxB = 1:

1. A*B*A = A
--------------------
A^2 x B = A --> B = 1/A and hence AxB = 1
OR A = 0 and thus AxB = 0

statement 1 is insufficient

2. B*A*B = B
-----------------
B^2 x A = B --> A = 1/B and hence AxB = 1
OR B = 0 and thus AxB = 0

A^2 x B = A --> B = 1/A

(1) and (2) together
------------------------
still insufficient as we have multiple cases or options

08 Jun 2007, 09:25
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