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Number theory Difficulty 5.5 AIME, harder Prove it North Macedonia

Find all the prime numbers pp for which there exist positive integers xx and yy that satisfy the equation
x(y2p)+y(x2p)=5p x(y^2 - p) + y(x^2 - p) = 5p

Solution

Given equation is equivalent to
(x+y)(xyp)=5p. (x + y)(xy - p) = 5p.
We consider the following cases:
1. Let x+y=1x + y = 1 and xy=6pxy = 6p. For prime p2p \geq 2 the equation x2x+6p=0x^2 - x + 6p = 0 has no solutions.

2. Let x+y=5x + y = 5 and xy=2pxy = 2p. For prime p2p \geq 2 the equation x5x+2p=0x - 5x + 2p = 0 has the determinant Δ=p24p20\Delta = p^2 - 4p - 20. The inequality 258p025 - 8p \geq 0 implies p{2,3}p \in \{2, 3\}. For p=2p = 2 we obtain the solutions (1,4)(1, 4) and (4,1)(4, 1).
For p=3p = 3 we obtain the solutions (2,3)(2, 3) and (3,2)(3, 2).

3. Let x+y=px + y = p and xy=p+5xy = p + 5. For prime p2p \geq 2 the equation x2px+p+5=0x^2 - px + p + 5 = 0 has the diskriminant Δ=p24p20\Delta = p^2 - 4p - 20. The inequality p24p200p^2 - 4p - 20 \geq 0 implies p7p \geq 7.
Let p24p20=q2p^2 - 4p - 20 = q^2 with 1qp1 \leq q \leq p. We obtain the equation (p2)2q2=24(p-2)^2 - q^2 = 24 which is equivalent to
(p+q2)(pq2)=24. (p + q - 2)(p - q - 2) = 24.
It follows that both numbers p+q2p+q-2 and pq2p-q-2 shall be even. We have two subcases:
a) p+q2=12p+q-2=12 and pq2=2p-q-2=2. We have p=9p=9, which is no prime.
b) p+q2=6p+q-2=6 and pq2=4p-q-2=4. We obtain p=7p=7 and q=1q=1. The equation has the solutions (3,4)(3, 4) and (4,3)(4, 3).

4. Let x+y=5px + y = 5p and xy=p+1xy = p + 1. It follows that pNp \notin \mathbb{N}.
So, the equation has natural solutions only for p{2,3,7}p \in \{2, 3, 7\}.

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