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Algebra Difficulty 5.4 AIME, harder Prove it

Let a,b,c a, b, c be positive real numbers for which a+b+c=1 a+b+c=1 . Prove that

abca+bc+bcab+ca+cabc+ab32. \frac{a-b c}{a+b c}+\frac{b-c a}{b+c a}+\frac{c-a b}{c+a b} \leq \frac{3}{2} .

Solution

Note that

1abca+bc=2bc1bc+bc=2bc(1b)(1c) 1-\frac{a-b c}{a+b c}=\frac{2 b c}{1-b-c+b c}=\frac{2 b c}{(1-b)(1-c)}

The inequality is equivalent to

2bc(1b)(1c)+2ca(1c)(1a)+2ab(1a)(1b)32. \frac{2 b c}{(1-b)(1-c)}+\frac{2 c a}{(1-c)(1-a)}+\frac{2 a b}{(1-a)(1-b)} \geq \frac{3}{2} .

Manipulation yields the equivalent

4(bc+ca+ab3abc)3(bc+ca+ab+1abcabc). 4(b c+c a+a b-3 a b c) \geq 3(b c+c a+a b+1-a-b-c-a b c) .

This simplifies to ab+bc+ca9abca b+b c+c a \geq 9 a b c or

1a+1b+1c9 \frac{1}{a}+\frac{1}{b}+\frac{1}{c} \geq 9 \text {. }

This is a consequence of the harmonic-arithmetic means inequality.

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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.