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Bunuel
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Super conceptually strong question & a high-quality one!
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Shouldn't the answer be 0. If y=0 then X?Y can never be equal to 1
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Bunuel
Official Solution:

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

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


We are given that a certain function, denoted as '?', is defined for all numbers \(x\) and \(y\) such that \(x?y = xy - x - y\).

Since also given that \(x?y = 1\), then \(xy - x - y = 1\).

Express \(x\) in terms of \(y\) to get \(x = \frac{y + 1}{y - 1}\).

If we substitute \(y = 1\) into this equation, the denominator becomes zero, leading to an undefined expression (as we cannot divide by zero). Therefore, \(y\) cannot be equal to 1.


Answer: D
Shouldn't the answer be 0. If y=0 then X?Y can never be equal to 1

No. If y = 0, then x?y = x?0 = x*0 - x - 0 = -x. For this to equal 1, -x must be 1, which means x = -1.
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It took me little longer to come to D, as I tested the values -

xy-x-y=1

Taking Y = 1 (Option D) makes the equation illogical
x-x-1=1
=> -1 = 1
Hence, Y cannot be 1.


Bunuel
Official Solution:

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

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


We are given that a certain function, denoted as '?', is defined for all numbers \(x\) and \(y\) such that \(x?y = xy - x - y\).

Since also given that \(x?y = 1\), then \(xy - x - y = 1\).

Express \(x\) in terms of \(y\) to get \(x = \frac{y + 1}{y - 1}\).

If we substitute \(y = 1\) into this equation, the denominator becomes zero, leading to an undefined expression (as we cannot divide by zero). Therefore, \(y\) cannot be equal to 1.


Answer: D
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I like the solution - it’s helpful.
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I like the solution - it’s helpful.
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I like the solution - it’s helpful.
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A quicker way to solve this; 20s Approach

1= xy-x-y >> 1= x(y-1)-y >> Now is there a way to eliminate x term. YES, but how? Put y=1 so that x(y-1) becomes 0. In that case we will be left with 1=-y and since we assumed y=1 to eliminate x terms, 1=-1 "NOT POSSIBLE". 20s and you're home. Thanks
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