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Algebra Difficulty 7.3 National olympiad, round 2 Find the answer

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

A number or a short expression. Spacing and $ signs are ignored.

Solution

To find all triples (x,y,z)(x, y, z) of positive integers such that xyzx \leq y \leq z and
x3(y3+z3)=2012(xyz+2),x^3(y^3 + z^3) = 2012(xyz + 2),
we proceed as follows:

First, note that 20122=235032012 \cdot 2 = 2^3 \cdot 503. Taking the equation modulo xx, we see that x2012x \mid 2012. Therefore, xx can be 1,2,4,503,1006,20121, 2, 4, 503, 1006, 2012. We will consider the feasible values of xx under the constraint xyzx \leq y \leq z.

### Case 1: x=1x = 1
The equation becomes:
y3+z3=2012(yz+2).y^3 + z^3 = 2012(yz + 2).
Factoring the left-hand side, we get:
(y+z)(y2yz+z2)=2012(yz+2).(y + z)(y^2 - yz + z^2) = 2012(yz + 2).
For 503y+z503 \mid y + z or 503y2yz+z2503 \mid y^2 - yz + z^2, we apply the lemma that 503y2yz+z2503 \nmid y^2 - yz + z^2 for positive integers yy and zz not both divisible by 503. Therefore, 503y+z503 \mid y + z. Let y+z=503ny + z = 503n.

Rewriting the equation:
n(y2yz+z2)=4(yz+2).n(y^2 - yz + z^2) = 4(yz + 2).
If n4n \geq 4, then (zy)22(z - y)^2 \leq 2, implying z=yz = y or z=y+1z = y + 1. Both cases lead to contradictions. Thus, n<4n < 4.

Since yy and zz have the same parity, n=2n = 2 is required, leading to y+z=1006y + z = 1006. This results in:
(y+z)2=10062=5yz+4,(y + z)^2 = 1006^2 = 5yz + 4,
which is a contradiction modulo 5. Hence, no solutions exist for x=1x = 1.

### Case 2: x=2x = 2
The equation becomes:
y3+z3=503(yz+1).y^3 + z^3 = 503(yz + 1).
Letting y+z=503ny + z = 503n, we rewrite this as:
n(y2yz+z2)=yz+1.n(y^2 - yz + z^2) = yz + 1.
If n3n \geq 3, then n(zy)2=1+(1n)yz<0n(z - y)^2 = 1 + (1 - n)yz < 0, a contradiction.

For n=1n = 1, we have:
y2yz+z2=yz+1,y^2 - yz + z^2 = yz + 1,
or (zy)2=1(z - y)^2 = 1, leading to z=y+1z = y + 1. Given y+z=503y + z = 503, we find y=251y = 251 and z=252z = 252. Verifying, (x,y,z)=(2,251,252)(x, y, z) = (2, 251, 252) satisfies the original equation.

For n=2n = 2, we get:
2(y2yz+z2)=yz+1,2(y^2 - yz + z^2) = yz + 1,
or 2(zy)2=1yz2(z - y)^2 = 1 - yz, forcing y=z=1y = z = 1, which contradicts y+z=1006y + z = 1006.

Thus, the only solution is (x,y,z)=(2,251,252)(x, y, z) = (2, 251, 252).

The answer is: (2,251,252)\boxed{(2, 251, 252)}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.