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simple one

by magical cook » Tue Jan 22, 2008 9:35 pm
For any numbers a and b, a · b = a + b – ab.
If a · b = 0, which of the following CANNOT be a value of b?
(A) 2
(B) 1
(C) 0
(D) -1
(E) -2/3


simple but I got it wong... OA will follow. can you pls explain?
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by simplyjat » Tue Jan 22, 2008 9:56 pm
The best way is to substitute all the answer choices for the value of B in A + B - AB = 0 and try to find out the value of A...

For the second answer choice we get
=> A + 1 - A = 0
=> 1 = 0
thus B cannot have a value 1.
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by sibbineni » Wed Jan 23, 2008 2:46 am
IMO B

Given Data:

a.b=a+b-ab

a.b=0

Now substitute all the choices in a+b-ab=0 ---(1)(since a.b=0)

1---> option b =2 subtitiuting in the equation 1 we have a=2

2---> option b=1 in equation 1 we have 1=0 so b cannot have a value

3---> option b=0 in equation we have a=0


4-->option b=-1 we have a=1/2


5--->option b=-2/3 we have a=2/5



in the (B) case we dont have a value......



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