Olympiad Maths Prep

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Problem 1191

National olympiad, first round
Number theory Difficulty 6.3 Find the answer

Of all positive integral solutions (x,y,z)(x,y,z) to the equation x3+y3+z33xyz=607,x^3+y^3+z^3-3xyz=607, compute the minimum possible value of x+2y+3z.x+2y+3z.

[i]Individual #7[/i]

Official solution

1. We start with the given equation:
x3+y3+z33xyz=607 x^3 + y^3 + z^3 - 3xyz = 607
This can be factored using the identity for the sum of cubes:
x3+y3+z33xyz=(x+y+z)(x2+y2+z2xyyzzx) x^3 + y^3 + z^3 - 3xyz = (x+y+z)(x^2 + y^2 + z^2 - xy - yz - zx)
Therefore, we have:
(x+y+z)(x2+y2+z2xyyzzx)=607 (x+y+z)(x^2 + y^2 + z^2 - xy - yz - zx) = 607

2. Since 607 is a prime number, the factors of 607 are 1 and 607. Thus, we have two cases to consider:
x+y+z=607andx2+y2+z2xyyzzx=1 x+y+z = 607 \quad \text{and} \quad x^2 + y^2 + z^2 - xy - yz - zx = 1

3. Next, we multiply the second equation by 2:
2(x2+y2+z2xyyzzx)=21=2 2(x^2 + y^2 + z^2 - xy - yz - zx) = 2 \cdot 1 = 2
This simplifies to:
(xy)2+(yz)2+(zx)2=2 (x-y)^2 + (y-z)^2 + (z-x)^2 = 2

4. Since the sum of three squares is 2, the only possible values for the squares are 1, 1, and 0. Therefore, two of the variables must differ by 1, and the third must be equal to one of the other two. Without loss of generality, assume:
x=yandz=x+1 x = y \quad \text{and} \quad z = x + 1

5. Substitute z=x+1z = x + 1 and y=xy = x into the first equation:
x+x+(x+1)=607 x + x + (x + 1) = 607
Simplify to find xx:
3x+1=607    3x=606    x=202 3x + 1 = 607 \implies 3x = 606 \implies x = 202

6. Therefore, we have:
x=202,y=202,z=203 x = 202, \quad y = 202, \quad z = 203

7. To find the minimum value of x+2y+3zx + 2y + 3z, substitute the values of xx, yy, and zz:
x+2y+3z=202+2(202)+3(203) x + 2y + 3z = 202 + 2(202) + 3(203)
Calculate step-by-step:
x+2y+3z=202+404+609=1215 x + 2y + 3z = 202 + 404 + 609 = 1215

The final answer is 1215\boxed{1215}

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.