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Algebra Difficulty 7.2 National olympiad, round 2 Prove it

65. (48th International Mathematical Olympiad China National Training Team Test Question, March 2007) Let positive real numbers u,v,wu, v, w satisfy u+v+w+uvw=4u+v+w+\sqrt{u v w}=4. Prove that
vwu+uwv+uvwu+v+w\sqrt{\frac{v w}{u}}+\sqrt{\frac{u w}{v}}+\sqrt{\frac{u v}{w}} \geqslant u+v+w

Solution

65. Proof: Let vwu=x,uwv=y,uvw=z\sqrt{\frac{v w}{u}}=x, \sqrt{\frac{u w}{v}}=y, \sqrt{\frac{u v}{w}}=z, then the original proposition is equivalent to: x,y,zR+x, y, z \in \mathbf{R}^{+}, and yz+zx+xy+xyz=4y z+z x+x y+x y z=4, then
x+y+zyz+zx+xyx+y+z \geqslant y z+z x+x y

The proof of equation (1) can be found in Exercise 64 (1) of this chapter.
Attachment: The solution provided by the National Training Team for this problem (provided by the problem group).
Proof: Let u=xy,v=yz,w=xz,x,y,zu=x y, v=y z, w=x z, x, y, z be positive real numbers, then the condition becomes
xyz+xy+yz+zx=4x y z+x y+y z+z x=4

The conclusion becomes
x+y+zxy+yz+zxx+y+z \geqslant x y+y z+z x

Since x,y,zx, y, z must have two numbers on the same side of 1, without loss of generality, assume y,zy, z are on the same side of 1, then
(y1)(z1)0(y-1)(z-1) \geqslant 0

Thus,
x(yz+y+z)=4yz>0x(y z+y+z)=4-y z>0

From this, we get
x=4yzyz+y+z4yzyz+2yz=(2+yz)(2yz)yz(yz+2)=2yzyzx=\frac{4-y z}{y z+y+z} \leqslant \frac{4-y z}{y z+2 \sqrt{y z}}=\frac{(2+\sqrt{y z})(2-\sqrt{y z})}{\sqrt{y z}(\sqrt{y z}+2)}=\frac{2-\sqrt{y z}}{\sqrt{y z}}

Therefore,
(x+1)yz2(x+1) \sqrt{y z} \leqslant 2

Notice that we need to prove
 Equation (※) xxzxy+xyzyzyz+xyzx(1y)(1z)yz(x+1)yz\begin{array}{l} \text { Equation (※) } \Leftrightarrow x-x z-x y+x y z \geqslant y z-y-z+x y z \Leftrightarrow \\ x(1-y)(1-z) \geqslant y z(x+1)-y-z \end{array}

Next, we prove Equation (※※). By Equation (1), the left side 0\geqslant 0; by Equation (2),
the right side yz2yz=(yz)20\leqslant \sqrt{y z} \cdot 2-y-z=-(\sqrt{y}-\sqrt{z})^{2} \leqslant 0 hence Equation (※※) holds.

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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.