For any numbers x and y, x#y = xy - x - y if x#y=1 which

Question

For any numbers  x  and  y ,  x@y=xy-x-y . If  x@y=1 , which of the following cannot be the value of  y  ?

A. -2
B. -1
C. 0
D. 1
E. 2

M23-33

Bunuel · Accepted Answer

Given  xy-x-y=1 , which is the same as  (1-x)(1-y)-1=1  or  (1-x)(1-y)=2 . Now, if  y=1  then  (1-x)(1-1)=0
eq{2} , so in order the given equation to hold true  y  cannot equal to 1.

Answer; D.

khamdy71 · Answer

SmartSelectImage_2016-07-26-09-42-18 Sent from my SM-N920R4 using GMAT Club Forum mobile app

walker · Answer

D xy-x-y=1 ==> y(x-1)-x=1 ==> y(x-1)=(x+1) ==> y=(x+1)/(x-1) ==> y=(x-1+2)/(x-1) ==> y=1+2/(x-1) 2/(x-1)<>0 ==> y<>1

LM · Answer

Y= (X+1)/(X-1)

Now substitute the choices, For Y=1,

X+1 = X-1

which is not possible, that means Y=1 is not possible.

sayantanc2k · Answer

similarly x= (y+1)/(y-1)...denominator cannot be 0, hence y cannot be 1.

kanusha · Answer

D

x#y = xy - x - y where x#y = 1 
so, choosing x and y values from option x= 0 and x = 1 then
substitute
1 = 0* 1 - 0 - 1 = -1 , where equation is # if y = 1 ,

seeing with x= 0 and y = -1 ,then yes 1 = 1, same as other too..

prsnt11 · Answer

Hey Bunuel,
What are the guidelines to factor  xy-x-y=1  as  (1-x)(1-y)-1 ? Can you please provide a link of study or similar problems? I'm unable to understand.
Thanks,

happyface101 · Answer

I second this question - can someone please provide some guidance? Thanks!!

TeamGMATIFY · Answer

There are no guidelines as such to factorization of  xy-x-y=1  as  (1-x)(1-y)-1 
Since the question is asking us to find the values that y cannot take, we are trying to make an equation in terms of y.
If (1-x)(1-y) = 0. This means either x = 1 or y =1

By bringing the equation in the form  (1-x)(1-y)  = 2
We have proved that (1-x)(1-y) ≠ 0'

Hence y cannot take the value 1

Does this help?

happyface101 · Answer

This doesn't help bc I still don't know the thought process of turning xy−x−y=1 into (1−x)(1−y)−1. Everything else after that make sense to me.

sayantanc2k · Answer

There are practically four different methods to factorise second-order expressions:

1. Take out common factor
2. Grouping
3. Difference of two squares
4. Middle term breaking

This particular example falls under category 2. If you would like further detailing on these 4 methods, you may either refer to a Maths text book of class 8 or write back to me.

LogicGuru1 · Answer

1 = xy - x - y
take y to LHS and take 1 to RHS
y = xy - x -1
y=x(y-1)-1
Pay attention to the term (y-1) ; if we take the value of y = 1 as given in option D then the term (y-1) will become 0 
y=x(y-1)-1 {take y=1}
1=x(1-1)-1
1=x(0)-1
1=-1 THIS DOESNT MAKE SENSE. 
SO Y = 1 cannot be the value of y

ANSWER IS D

bumpbot · Answer

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For any numbers x and y, x#y = xy - x - y if x#y=1 which

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