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Algebra Difficulty 6.3 National olympiad Prove it

4.6.11 ** Let a,b,ca, b, c be positive real numbers, and satisfy abc=1a b c=1, prove: (a1+1b)(b1+1c)(c1+1a)1\left(a-1+\frac{1}{b}\right)\left(b-1+\frac{1}{c}\right)\left(c-1+\frac{1}{a}\right) \leqslant 1.

Solution

Analysis 1
b1+1c=b(11b+1bc)=b(1+a1b), b-1+\frac{1}{c}=b\left(1-\frac{1}{b}+\frac{1}{b c}\right)=b\left(1+a-\frac{1}{b}\right),

Therefore,
(a1+1b)(b1+1c)=b(a1+1b)(a+11b)=b[a2(11b)2]ba2, \begin{aligned} \left(a-1+\frac{1}{b}\right)\left(b-1+\frac{1}{c}\right) & =b\left(a-1+\frac{1}{b}\right)\left(a+1-\frac{1}{b}\right) \\ & =b\left[a^{2}-\left(1-\frac{1}{b}\right)^{2}\right] \leqslant b a^{2}, \end{aligned}

Similarly, (b1+1c)(c1+1a)cb2,(c1+1a)(a1+1b)ac2\left(b-1+\frac{1}{c}\right)\left(c-1+\frac{1}{a}\right) \leqslant c b^{2},\left(c-1+\frac{1}{a}\right)\left(a-1+\frac{1}{b}\right) \leqslant a c^{2}. If a1+1b,b1+1c,c1+1aa-1+\frac{1}{b}, b-1+\frac{1}{c}, c-1+\frac{1}{a} are not all positive, without loss of generality, assume a1+1b0a-1+\frac{1}{b} \leqslant 0, then a11b0,c1+1a>0a \leqslant 1-\frac{1}{b}0, c-1+\frac{1}{a}>0, hence the proposition holds.
If a1+1b,b1+1c,c1+1aa-1+\frac{1}{b}, b-1+\frac{1}{c}, c-1+\frac{1}{a} are all positive, then from the above three inequalities we get
(a1+1b)2(b1+1c)2(c1+1a)2a3b3c3=1,(a1+1b)(b1+1c)(c1+1a)1. \begin{array}{l} \left(a-1+\frac{1}{b}\right)^{2}\left(b-1+\frac{1}{c}\right)^{2}\left(c-1+\frac{1}{a}\right)^{2} \leqslant a^{3} b^{3} c^{3}=1, \\ \left(a-1+\frac{1}{b}\right)\left(b-1+\frac{1}{c}\right)\left(c-1+\frac{1}{a}\right) \leqslant 1 . \end{array}

Analysis 2
By abc=1a b c=1, without loss of generality, let a=xy,b=yz,c=zxa=\frac{x}{y}, b=\frac{y}{z}, c=\frac{z}{x}, where x,y,zR+x, y, z \in \mathbf{R}^{+}. Then the inequality to be proved is
(xy1+zy)(yz1+xz)(zx1+yx)1, \left(\frac{x}{y}-1+\frac{z}{y}\right)\left(\frac{y}{z}-1+\frac{x}{z}\right)\left(\frac{z}{x}-1+\frac{y}{x}\right) \leqslant 1,

which is
(x+yz)(y+zx)(z+xy)xyz. (x+y-z)(y+z-x)(z+x-y) \leqslant x y z .

Since the sum of any two of x+yz,y+zx,z+xyx+y-z, y+z-x, z+x-y is positive, at most one of them can be negative. If exactly one is negative, then (1) holds; if all three are non-negative, then
x2x2(yz)2=(x+yz)(x+zy),y2(y+zx)(x+yz),z2(z+xy)(y+zx), \begin{array}{l} x^{2} \geqslant x^{2}-(y-z)^{2}=(x+y-z)(x+z-y), \\ y^{2} \geqslant(y+z-x)(x+y-z), z^{2} \geqslant(z+x-y)(y+z-x), \end{array}

Multiplying these three inequalities and taking the square root, we get (x+yz)(y+zx)(z+xy)xyz(x+y-z)(y+z-x)(z+x-y) \leqslant x y z. Hence the proposition is proved.

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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.