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Number theory Difficulty 5.6 AIME, harder Prove it Czech-Polish-Slovak Mathematical Match

Determine all pairs of integers (x,y)(x, y) satisfying the equation
y(x+y)=x37x2+11x3. y(x + y) = x^3 - 7x^2 + 11x - 3.

Solution

The considered equation is equivalent to
(2y+x)2=4x327x2+44x12=(x2)(4x219x+6)=(x2)((x2)(4x11)16). (2y + x)^2 = 4x^3 - 27x^2 + 44x - 12 = (x - 2)(4x^2 - 19x + 6) = (x - 2)((x - 2)(4x - 11) - 16).
The expression above must be a perfect square. Therefore we have either x=2x = 2 (and y=1y = -1), or (x2)=ks2(x - 2) = ks^2, where k{2,1,1,2}k \in \{-2, -1, 1, 2\} and sNs \in \mathbb{N}; indeed, if for some prime pp and nonnegative integer mm the number p2m+1p^{2m+1} divides (x2)(x-2) but p2m+2p^{2m+2} does not, then we have p(x2)(4x11)16p \mid (x-2)(4x-11) - 16, so p16p \mid 16 and p=2p = 2.
We will consider three cases separately.

The case of k=±2k = \pm 2. Then we have 4x219x+6=±2u24x^2 - 19x + 6 = \pm 2u^2 for some integer uu, or, equivalently,
(8x19)2265=±32u2, (8x - 19)^2 - 265 = \pm 32u^2,
a contradiction modulo 55.

The case of k=1k = 1. Then 4x219x+6=u24x^2 - 19x + 6 = u^2 for some integer uu, which leads to
265=(8x19)216u2=(8x194u)(8x19+4u). 265 = (8x - 19)^2 - 16u^2 = (8x - 19 - 4u)(8x - 19 + 4u).
We easily check that x=6x = 6 is the only solution of this equation (we simply consider all possible decompositions: 265=1265=553=265 = 1 \cdot 265 = 5 \cdot 53 = \dots, etc, and take into account the fact that x2=s2x - 2 = s^2). Therefore, we obtain two solutions of the original equation: (x,y){(6,3),(6,9)}(x, y) \in \{(6, 3), (6, -9)\}.

The case of k=1k = -1. As before, we have 4x219x+6=u24x^2 - 19x + 6 = -u^2, which is equivalent to
265=(8x19)2+(4u)2 265 = (8x - 19)^2 + (4u)^2
and we check all possibilities with u4u \le 4: for u=0,1,2u = 0, 1, 2 there are no solutions. If u=3u = 3, then we obtain (8x19)2=121=112(8x - 19)^2 = 121 = 11^2, which leads to x=1x = 1, which gives two solutions: (x,y){(1,1),(1,2)}(x, y) \in \{(1, 1), (1, -2)\}. Finally, for u=4u = 4 we arrive at (8x19)2=9=32(8x - 19)^2 = 9 = 3^2 and x=2x = 2, which gives (x,y)=(2,1)(x, y) = (2, -1).

Therefore, the set of solutions is as follows:

\{(6, 3), (6, -9), (1, 1), (1, -2), (2, -1)\}.

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