Maths Olympiad Prep

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Number theory Difficulty 5.7 AIME, harder Prove it Taiwan

Find all triples of positive integers (x,y,z)(x, y, z) such that xyzx \le y \le z and
x3(y3+z3)=2012(xyz+2). x^3(y^3 + z^3) = 2012(xyz + 2).

Solution

First note that xx divides 20122=235032012 \cdot 2 = 2^3 \cdot 503. If 503x503 \mid x then the right-hand side of the equation is divisible by 5033503^3, hence 5032xyz+2503^2 \mid xyz + 2. Since 503x503 \mid x, this is a contradiction. Therefore x=2m,m{0,1,2,3}x = 2^m, m \in \{0, 1, 2, 3\}. If m2m \ge 2 then 262012(xyz+2)2^6 \mid 2012(xyz + 2). However, the highest power of 22 dividing 20122012 is 222^2, and the highest power of 22 dividing xyz+2=2myz+2xyz + 2 = 2^m yz + 2 is 212^1. So x=1x = 1 or x=2x = 2 produces the following two equations
y3+z3=2012(yz+2)andy3+z3=503(yz+1). y^3 + z^3 = 2012(yz + 2) \quad \text{and} \quad y^3 + z^3 = 503(yz + 1).
In both of the equations above, we obtain that the prime 503=3167+2503 = 3 \cdot 167 + 2 divides y3+z3y^3 + z^3. We claim that 503y+z503 \mid y+z. If 503y503 \mid y, then 503y+z503 \mid y+z is obvious. So we assume that 503y503 \nmid y and 503z503 \nmid z. Then by Fermat's Little Theorem we get y502z502(mod503)y^{502} \equiv z^{502} \pmod{503}. On the other hand, y3z3(mod503)y^3 \equiv -z^3 \pmod{503} implies y3167z3167(mod503)y^{3 \cdot 167} \equiv -z^{3 \cdot 167} \pmod{503}, that is, y501z501(mod503)y^{501} \equiv -z^{501} \pmod{503}. Hence we obtain yz(mod503)y \equiv -z \pmod{503}; that is, 503y+z503 \mid y+z.
Therefore y+z=503k,k1y+z=503k, k \ge 1. In view of y3+z3=(y+z)((yz)2+yz)y^3+z^3 = (y+z)((y-z)^2+yz), the two equations above can respectively be put into the following forms
k(yz)2+(k4)yz=8,(1) k(y - z)^2 + (k - 4)yz = 8, \qquad (1)
k(yz)2+(k1)yz=1.(2) k(y - z)^2 + (k - 1)yz = 1. \qquad (2)
In (1), we have (k4)yz8(k-4)yz \le 8, from which we get k4k \le 4. Indeed if k>4k > 4, then 1(k4)yz81 \le (k-4)yz \le 8, which forces y8y \le 8 and z8z \le 8. This is impossible, because y+z=503k503y+z = 503k \ge 503. Next note that in the first equation y3+z3y^3+z^3 is even. Therefore y+z=503ky+z = 503k is also even, meaning that kk is even. So k=2k=2 or k=4k=4. Clearly when k=4k=4, equation (1) has no integer solutions. If k=2k=2 then equation (1) can be written as (y+z)25yz=4(y+z)^2 - 5yz = 4. Since y+z=503k=5032y+z = 503k = 503 \cdot 2, we get 5yz=50322245yz = 503^2 \cdot 2^2 - 4. However 5032224503^2 \cdot 2^2 - 4 is not a multiple of 55. Therefore equation (1) has no integer solutions.
In (2), we have 0(k1)yz10 \le (k-1)yz \le 1, from which we get k=1k=1 or k=2k=2. Also 0k(yz)210 \le k(y-z)^2 \le 1, so k=2k=2 only when y=zy=z. In that case y=z=1y=z=1, which is false since y+z503y+z \ge 503. Therefore k=1k=1 and equation (2) can be written as (yz)2=1(y-z)^2 = 1, giving zy=yz=1z-y = |y-z| = 1. From k=1k=1 and y+z=503ky+z = 503k, we deduce y=251,z=252y = 251, z = 252. In summary, the triple (2, 251, 252) is the unique solution.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from the original; metadata (topic, difficulty) added by this project.