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Algebra Difficulty 6.3 National olympiad Prove it Saudi Arabia

Suppose that x,y,zx, y, z are non-zero real numbers such that
x=2yz,y=2zx,z=2xy x = 2 - \frac{y}{z}, \quad y = 2 - \frac{z}{x}, \quad z = 2 - \frac{x}{y}
Find all possible values of T=x+y+zT = x + y + z.

Solution

From the given conditions, we have
xz=2zy,xy=2xz,yz=2yx. xz = 2z - y, \quad xy = 2x - z, \quad yz = 2y - x.
Taking the sum of these equations, side by side, we have
xy+yz+zx=x+y+z=T. xy + yz + zx = x + y + z = T.
From xz=2zyxz = 2z - y, we also can get 2zxy=xzx3zT=x(z1)2z - x - y = xz - x \rightarrow 3z - T = x(z - 1). Make the similar equations and multiply them, we get
xyz(x1)(y1)(z1)=(3xT)(3yT)(3zT). xyz(x - 1)(y - 1)(z - 1) = (3x - T)(3y - T)(3z - T).
Note that (x1)(y1)(z1)=xyz(xy+yz+zx)+(x+y+z)1=xyz1(x - 1)(y - 1)(z - 1) = xyz - (xy + yz + zx) + (x + y + z) - 1 = xyz - 1 and
(3xT)(3yT)(3zT)=27xyz9T(xy+yz+zx)+T2(3x+3y+3z)T3=27xyz9T2+2T3 \begin{aligned} (3x - T)(3y - T)(3z - T) &= 27xyz - 9T(xy + yz + zx) + T^2(3x + 3y + 3z) - T^3 \\ &= 27xyz - 9T^2 + 2T^3 \end{aligned}
Thus
xyz(xyz1)=27xyz9T2+2T3 or (xyz)228xyz=2T39T2. \begin{equation*} xyz(xyz - 1) = 27xyz - 9T^2 + 2T^3 \text{ or } (xyz)^2 - 28xyz = 2T^3 - 9T^2. \tag{1} \end{equation*}
By the similar transformation, xz=2z+yxz+2x=2z+2xyx(z+2)=2T3yxz = 2z + y \rightarrow xz + 2x = 2z + 2x - y \rightarrow x(z + 2) = 2T - 3y. Make the similar equations and multiply them, side by side, we get
(xyz)2+6(xyz)T+35xyz=18T24T3. (xyz)^2 + 6(xyz)T + 35xyz = 18T^2 - 4T^3.
On the other hand, xz=2zyxyz=2yzy2xz = 2z - y \rightarrow xyz = 2yz - y^2. Make the similar equations and multiply them, side by side, we get
3xyz=2(xy+yz+zx)(x2+y2+z2)=4(xy+yz+zx)(x+y+z)2=4TT23xyz = 2(xy + yz + zx) - (x^2 + y^2 + z^2) = 4(xy + yz + zx) - (x + y + z)^2 = 4T - T^2.
So we have xyz=4TT23xyz = \frac{4T - T^2}{3}, then by substituting to equation (1), we get
(4TT23)228(4TT23)=2T39T2 \begin{equation*} \left(\frac{4T - T^2}{3}\right)^2 - 28\left(\frac{4T - T^2}{3}\right) = 2T^3 - 9T^2 \tag{2} \end{equation*}
Solving this equation, we have T{0,3,7,16}T \in \{0, 3, 7, 16\}.

1. If T=0T = 0, then xyz=0xyz = 0, a contradiction.
2. If T=16T = 16, then xyz=64xyz = -64 and this pair (T,xyz)=(16,64)(T, xyz) = (16, -64) does not satisfy equation (2).
3. If T=3T = 3, then xyz=1xyz = 1 and by Vieta's theorem, x,y,zx, y, z are real roots of t33t2+3t1=0t^3 - 3t^2 + 3t - 1 = 0, which implies that x=y=z=1x = y = z = 1.
4. If T=7T = 7, we get xyz=7xyz = -7 and x,y,zx, y, z are roots of t37t2+7t+7=0t^3 - 7t^2 + 7t + 7 = 0, this also have three distinct real roots.

Hence, all possible values of TT are 33 or 77.

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