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(x−1)(1−y)=?

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(x−1)(1−y)=?

by Vincen » Wed Jan 24, 2018 8:36 am
(x−1)(1−y)=?

(1) x+y=xy

(2) x−y=0

The OA is the option A.

How can I conclude that the correct answer is A? I think it should be B. Experts, can you help me? Thanks in advanced.
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by DrMaths » Thu Jan 25, 2018 7:32 am
(x−1)(1−y)=-(x-1)(y-1) = -(xy - x - y + 1) = 1 - xy + x + y

(1) Substitute x+y=xy to get 1 - xy + xy = 1 SUFFICIENT
(2) Substitute x=y to get 1 - x^2 + 2x INSUFFICIENT
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