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Algebra Difficulty 7.1 National olympiad, round 2 Prove it

22. When x,y,z[1,2]x, y, z \in [1,2], prove the following inequality holds, and specify the conditions under which equality holds:
(cycx)(cyc1x)6(cycxy+z).\left(\sum_{\text{cyc}} x\right)\left(\sum_{\text{cyc}} \frac{1}{x}\right) \geqslant 6\left(\sum_{\text{cyc}} \frac{x}{y+z}\right).

Solution

22. First, we have
(cycx)(cyc1x)9=cyc(xy)2xy6(cycxy+z)9=cyc3(xy)2(x+z)(y+z)\begin{array}{c} \left(\sum_{\mathrm{cyc}} x\right)\left(\sum_{\mathrm{cyc}} \frac{1}{x}\right)-9=\sum_{\mathrm{cyc}} \frac{(x-y)^{2}}{x y} \\ 6\left(\sum_{\mathrm{cyc}} \frac{x}{y+z}\right)-9=\sum_{\mathrm{cyc}} \frac{3(x-y)^{2}}{(x+z)(y+z)} \end{array}

From equations (7) and (8), to prove the original inequality, we only need to prove
cyc(xy)2xycyc3(xy)2(x+z)(y+z)\sum_{\mathrm{cyc}} \frac{(x-y)^{2}}{x y} \geqslant \sum_{\mathrm{cyc}} \frac{3(x-y)^{2}}{(x+z)(y+z)}

Let
Sx=1yz3(y+x)(z+x)Sy=1zx3(z+y)(x+y)Sz=1xy3(x+z)(y+z)\begin{array}{l} S_{x}=\frac{1}{y z}-\frac{3}{(y+x)(z+x)} \\ S_{y}=\frac{1}{z x}-\frac{3}{(z+y)(x+y)} \\ S_{z}=\frac{1}{x y}-\frac{3}{(x+z)(y+z)} \end{array}

Assume without loss of generality that xyzx \geqslant y \geqslant z. Clearly, Sx0S_{x} \geqslant 0. Since y+zxy+z \geqslant x, it is easy to see that xy+zy+y22xz0x y+z y+ y^{2}-2 x z \geqslant 0, which means Sy0S_{y} \geqslant 0. If Sz0S_{z} \geqslant 0, the inequality is proved. If Sz0S_{z} \leqslant 0, we can prove that Sy+Sz0S_{y}+S_{z} \geqslant 0.

In fact,
Sy+Sx=1xz3(x+y)(z+y)+1xy3(x+z)(z+y)=4yx2z+x2y2+2y2z2+y3x+y3z+x2z2+xz3+yz3xz(x+y)(z+y)y(x+z)\begin{aligned} & S_{y}+S_{x}=\frac{1}{x z}-\frac{3}{(x+y)(z+y)}+\frac{1}{x y}-\frac{3}{(x+z)(z+y)} \\ = & \frac{-4 y x^{2} z+x^{2} y^{2}+2 y^{2} z^{2}+y^{3} x+y^{3} z+x^{2} z^{2}+x z^{3}+y z^{3}}{x z(x+y)(z+y) y(x+z)} \end{aligned}

When x,y,z[1,2]x, y, z \in[1,2], the above expression is clearly greater than zero.
Since (xz)2(xy)2(x-z)^{2} \geqslant(x-y)^{2}, we have
cyc Sx(yz)2Sx(yz)2+(Sy+Sz)(xy)20\sum_{\text {cyc }} S_{x}(y-z)^{2} \geqslant S_{x}(y-z)^{2}+\left(S_{y}+S_{z}\right)(x-y)^{2} \geqslant 0

From equation (10), we know that equation (9) holds, i.e., the original inequality holds, with equality if and only if x=y=zx=y=z or x=2,y=z=1x=2, y=z=1.

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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.