Maths Olympiad Prep

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Algebra Difficulty 5.4 AIME, harder Prove it Brazil

Solve, in real numbers, the system
x+1y=y+1z=z+1xxyz=1 \begin{aligned} x + \frac{1}{y} = y + \frac{1}{z} = z + \frac{1}{x} \\ xyz = 1 \end{aligned}

Solution

Since xyz=1xyz = 1,
x+1y=y+1z    xyz+z=y2z+y    1y=z(1y2)    y=1 or z=11+y \begin{aligned} x + \frac{1}{y} &= y + \frac{1}{z} \iff xyz + z = y^2z + y \\ \iff 1 - y = -z(1 - y^2) \iff y = 1 \text{ or } z = -\frac{1}{1+y} \end{aligned}
If y=1y = 1, the system reduces to xz=1    z=1xxz = 1 \iff z = \frac{1}{x} and
x+1=1+1z=z+1x    x+1=1+x=2x    x=1 or x=2 x + 1 = 1 + \frac{1}{z} = z + \frac{1}{x} \iff x + 1 = 1 + x = \frac{2}{x} \iff x = 1 \text{ or } x = -2
If z=11+yz = -\frac{1}{1+y}, then xyz=1    x=1+yyxyz = 1 \iff x = -\frac{1+y}{y} and
1+yy+1y=y(1+y)=11+yy1+y -\frac{1+y}{y} + \frac{1}{y} = y - (1+y) = -\frac{1}{1+y} - \frac{y}{1+y}
(Solutions)
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which is true for every y0,1y \neq 0, -1 (all three expressions are equal to 1-1).
So all solutions are (1,1,1)(1, 1, 1) and (1+tt,t,11+t)(-\frac{1+t}{t}, t, -\frac{1}{1+t}) and its cyclic analogous triples. Notice that t=1t=1 yields the solution (2,1,12)(-2, 1, -\frac{1}{2}).

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.