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Number theory Difficulty 6.5 National olympiad Find the answer

Example 9 Find all positive integer tuples (a,b,c,x,y,z)(a, b, c, x, y, z) such that
{a+b+c=xyz,x+y+z=abc,\left\{\begin{array}{l} a+b+c=x y z, \\ x+y+z=a b c, \end{array}\right.

where abc,xyza \geqslant b \geqslant c, x \geqslant y \geqslant z.

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

Solution

By symmetry, we only need to consider the case xax \geqslant a. At this time,
xyz=a+b+c3a3xx y z=a+b+c \leqslant 3 a \leqslant 3 x

Thus,
yz3y z \leqslant 3

Therefore,
(y,z)=(1,1),(2,1),(3,1)(y, z)=(1,1),(2,1),(3,1)

When (y,z)=(1,1)(y, z)=(1,1), a+b+c=xa+b+c=x and x+2=abcx+2=a b c, thus
abc=a+b+c+2a b c=a+b+c+2

If c2c \geqslant 2, then
a+b+c+23a+24aabca+b+c+2 \leqslant 3 a+2 \leqslant 4 a \leqslant a b c

Equality holds if and only if a=b=c=2a=b=c=2.
If c=1c=1, then
ab=a+b+3a b=a+b+3
(a1)(b1)=4(a-1)(b-1)=4
(a,b)=(5,2),(3,3)(a, b)=(5,2),(3,3)

When (y,z)=(2,1)(y, z)=(2,1), 2abc=2x+6=a+b+c+62 a b c=2 x+6=a+b+c+6, similar discussion shows that c=1c=1, thus
(2a1)(2b1)=15(2 a-1)(2 b-1)=15

We get
(a,b)=(3,2)(a, b)=(3,2)

When (y,z)=(3,1)(y, z)=(3,1), 3abc=3x+12=a+b+c+123 a b c=3 x+12=a+b+c+12, similarly, there is no solution in this case.

In summary, we have
(a,b,c,x,y,z)=(2,2,2,6,1,1),(5,2,1,8,1,1),(3,3,1,7,1,1)(3,2,1,3,2,1),(6,1,1,2,2,2),(8,1,1,5,2,1)(7,1,1,3,3,1)\begin{aligned} & (a, b, c, x, y, z) \\ = & (2,2,2,6,1,1),(5,2,1,8,1,1),(3,3,1,7,1,1) \\ & (3,2,1,3,2,1),(6,1,1,2,2,2),(8,1,1,5,2,1) \\ & (7,1,1,3,3,1) \end{aligned}

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