AlgebraDifficulty 6.8National olympiadFind the answer
Find all triples of positive integers (x,y,z) that satisfy the equation \begin{align*} 2(x+y+z+2xyz)^2=(2xy+2yz+2zx+1)^2+2023 \end{align*}
A number or a short expression. Spacing and $ signs are ignored.
Solution
We claim that the only solutions are (2,3,3) and its permutations. Factoring the above squares and canceling the terms gives you: 8(xyz)2+2(x2+y2+z2)=4((xy)2+(yz)2+(zx)2)+2024 Jumping on the coefficients in front of the x2 , y2 , z2 terms, we factor into: (2x2−1)(2y2−1)(2z2−1)=2023 Realizing that the only factors of 2023 that could be expressed as (2x2−1) are 1 , 7 , and 17 , we simply find that the only solutions are (2,3,3) by inspection. -Max Alternatively, a more obvious factorization is: 2(x+y+z+2xyz)2=(2xy+2yz+2zx+1)2+2023 (2x+2y+2z+22xyz)2−(2xy+2yz+2zx+1)2=2023 (22xyz+2xy+2yz+2zx+2x+2y+2z+1)(22xyz−2xy−2yz−2zx+2x+2y+2z−1)=2023 (2x+1)(2y+1)(2z+1)(2x−1)(2y−1)(2z−1)=2023 (2x2−1)(2y2−1)(2z2−1)=2023 Proceed as above. ~eevee9406
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