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Algebra Difficulty 5.5 AIME, harder Find the answer

II. (50 points) Find the largest real number mm such that the inequality
1x+1y+1z+m1+x1+y+1+y1+z+1+z1+x \frac{1}{x}+\frac{1}{y}+\frac{1}{z}+m \leqslant \frac{1+x}{1+y}+\frac{1+y}{1+z}+\frac{1+z}{1+x}

holds for any positive real numbers xx, yy, zz satisfying xyz=x+y+z+2x y z=x+y+z+2.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let x=y=z=2x=y=z=2, we get m32m \leqslant \frac{3}{2}.
Below, we prove that the maximum value of mm is 32\frac{3}{2}.
Let x=1a1,y=1b1,z=1c1x=\frac{1}{a}-1, y=\frac{1}{b}-1, z=\frac{1}{c}-1, then we have
(1a1)(1b1)(1c1)=1a+1b+1c1 \left(\frac{1}{a}-1\right)\left(\frac{1}{b}-1\right)\left(\frac{1}{c}-1\right)=\frac{1}{a}+\frac{1}{b}+\frac{1}{c}-1 \text {. }

Rearranging gives a+b+c=1a+b+c=1.
Thus, the problem is transformed into proving:
a1a+b1b+c1c+32ba+cb+ac, i.e., 32(acab+c)+(babc+a)+(cbca+b), \begin{array}{c} \frac{a}{1-a}+\frac{b}{1-b}+\frac{c}{1-c}+\frac{3}{2} \leqslant \frac{b}{a}+\frac{c}{b}+\frac{a}{c}, \\ \text { i.e., } \frac{3}{2} \leqslant\left(\frac{a}{c}-\frac{a}{b+c}\right)+\left(\frac{b}{a}-\frac{b}{c+a}\right)+\left(\frac{c}{b}-\frac{c}{a+b}\right), \end{array}

which is equivalent to 32abc(b+c)+bca(c+a)+cab(a+b)\frac{3}{2} \leqslant \frac{a b}{c(b+c)}+\frac{b c}{a(c+a)}+\frac{c a}{b(a+b)}.
 and abc(b+c)+bca(c+a)+cab(a+b)=(ab)2abc(b+c)+(bc)2abc(c+a)+(a)2abc(a+b)(ab+bc+ca)22abc(a+b+c)=a2b2+b2c2+c2a2+2abc(a+b+c)2abc(a+b+c)abc(a+b+c)+2abc(a+b+c)2abc(a+b+c)=32. \begin{array}{l} \text { and } \frac{a b}{c(b+c)}+\frac{b c}{a(c+a)}+\frac{c a}{b(a+b)} \\ =\frac{(a b)^{2}}{a b c(b+c)}+\frac{(b c)^{2}}{a b c(c+a)}+\frac{(a)^{2}}{a b c(a+b)} \\ \geqslant \frac{(a b+b c+c a)^{2}}{2 a b c(a+b+c)} \\ =\frac{a^{2} b^{2}+b^{2} c^{2}+c^{2} a^{2}+2 a b c(a+b+c)}{2 a b c(a+b+c)} \\ \geqslant \frac{a b c(a+b+c)+2 a b c(a+b+c)}{2 a b c(a+b+c)}=\frac{3}{2} . \end{array}

In conclusion, the maximum value of mm is 32\frac{3}{2}.

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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.