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Algebra Difficulty 5.4 AIME, harder Prove it Czech Republic

Solve the system of equations
1xy=xz+1,1yz=yx+1,1zx=zy+1 \frac{1}{xy} = \frac{x}{z} + 1, \quad \frac{1}{yz} = \frac{y}{x} + 1, \quad \frac{1}{zx} = \frac{z}{y} + 1
in the domain of the real numbers.

Solution

From the form of the equations it is immediate that xyz0xyz \neq 0. Two of the numbers x,y,zx, y, z have to be of the same sign; then the right-hand side of the equation where the ratio of these two numbers occurs is positive, hence so must be the corresponding left-hand side, which implies that the third of the numbers x,y,zx, y, z must also have the same sign as the first and the second. Thus either x,y,z>0x, y, z > 0, or x,y,z<0x, y, z < 0. Let us consider only the former case (the latter can be reduced to it by passing from the solution (x,y,z)(x, y, z) to the solution (x,y,z)(-x, -y, -z)).

Multiply the first two equations of the system by the expression xyzxyz and then subtract them; this gives, upon a small manipulation, zx=y(x2yz)z - x = y(x^2 - yz). If a triple (x,y,z)(x, y, z) is a solution, then so are also the triples (y,z,x)(y, z, x) and (z,x,y)(z, x, y); thus we may assume that x=max{x,y,z}x = \max\{x, y, z\}. Then zx0z - x \le 0 and x2yz0x^2 - yz \ge 0 (remember that x,y,z>0x, y, z > 0), so the equality zx=y(x2yz)z - x = y(x^2 - yz), together with the condition y>0y > 0, implies that zx=x2yz=0z - x = x^2 - yz = 0, which means that x=y=zx = y = z. The system then reduces to the single equation 1/x2=1+11/x^2 = 1 + 1, which has a (unique) positive root x=2/2x = \sqrt{2}/2.

Conclusion. The system has exactly two solutions, x=y=z=±2/2x = y = z = \pm \sqrt{2}/2.

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