Trying to be pretty

1 x 2 + 1 x y + 1 y 2 = 1 \frac{1}{x^{2}}+\frac{1}{xy}+\frac{1}{y^{2}}=1

Find the number of integer solution(s) of the pair ( x , y ) (x,y) that satisfy the equation above.


The answer is 2.

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2 solutions

Abhishek Sinha
Jul 15, 2015

Simplifying, we have ( x + y ) 2 = x y ( 1 + x y ) , x 0 , y 0 (x+y)^2=xy(1+xy), \hspace {5pt} x\neq 0, y\neq 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 x+y=0, xy+1=0 This yields two solutions x = 1 , y = 1 x=1,y=-1 and x = 1 , y = 1 x=-1,y=1 .

Jessica Wang
Jul 15, 2015

Without loss of generality, assume x 2 y 2 x^{2}\geqslant y^{2} .

Then 3 y 2 1 x 2 + 1 x y + 1 y 2 = 1. \frac{3}{y^{2}}\geqslant \frac{1}{x^{2}}+\frac{1}{xy}+\frac{1}{y^{2}}=1.

Therefore y 2 3 y = ± 1 y^{2}\leqslant 3\: \Rightarrow y=\pm 1 .

When y = 1 y=1 , x = 1 x=-1 ;

When y = 1 y=-1 , x = 1 x=1 .

\therefore There are 2 \boxed{2} solutions.

Moderator note:

The first line previously made the assumption that x y x \geq y . Why won't that be sufficient? Why do we need x 2 y 2 x^2 \geq y^2 instead?

I beg your pardon! The assumption in the first line should be x 2 y 2 x^{2}\geqslant y^{2} .

Jessica Wang - 5 years, 11 months ago

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Right! Could you edit that into your solution? Thanks!

Calvin Lin Staff - 5 years, 11 months ago

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@Calvin Lin Edited! :)

Jessica Wang - 5 years, 11 months ago

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