x 2 1 + x y 1 + y 2 1 = 1
Find the number of integer solution(s) of the pair ( x , y ) that satisfy the equation above.
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Without loss of generality, assume x 2 ⩾ y 2 .
Then y 2 3 ⩾ x 2 1 + x y 1 + y 2 1 = 1 .
Therefore y 2 ⩽ 3 ⇒ y = ± 1 .
When y = 1 , x = − 1 ;
When y = − 1 , x = 1 .
∴ There are 2 solutions.
The first line previously made the assumption that x ≥ y . Why won't that be sufficient? Why do we need x 2 ≥ y 2 instead?
I beg your pardon! The assumption in the first line should be x 2 ⩾ y 2 .
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Right! Could you edit that into your solution? Thanks!
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Simplifying, we have ( x + y ) 2 = x y ( 1 + x y ) , x = 0 , y = 0 This implies a perfect square is equal to the product of two consective integers, which is impossible unless both sides equal to zero. Thus it follows that x + y = 0 , x y + 1 = 0 This yields two solutions x = 1 , y = − 1 and x = − 1 , y = 1 .