Maths Olympiad Prep

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Algebra Difficulty 6.6 National olympiad Prove it

Example 4 Let a,b,ca, b, c be positive numbers, prove that
c2a2a+b+a2b2b+c+b2c2c+a0\frac{c^{2}-a^{2}}{a+b}+\frac{a^{2}-b^{2}}{b+c}+\frac{b^{2}-c^{2}}{c+a} \geq 0

Solution

Analysis: This problem requires appropriately transforming the inequality to be proven, making the forms on both sides symmetrical, to create conditions for using the rearrangement inequality.

Proof: The inequality to be proven is equivalent to
c2a+b+a2b+c+b2c+aa2a+b+b2b+c+c2c+a,\frac{c^{2}}{a+b}+\frac{a^{2}}{b+c}+\frac{b^{2}}{c+a} \geq \frac{a^{2}}{a+b}+\frac{b^{2}}{b+c}+\frac{c^{2}}{c+a},

By the cyclic symmetry of the inequality, without loss of generality, assume abca \leq b \leq c, then
a2b2c2,a+ba+cb+ca^{2} \leq b^{2} \leq c^{2}, \quad a+b \leq a+c \leq b+c
1a+b1a+c1b+c,c2a+b+a2b+c+b2c+a is the sum in ascending order, a2a+b+b2b+c+c2c+a is the sum in random order,\frac{1}{a+b} \geq \frac{1}{a+c} \geq \frac{1}{b+c}, \quad \frac{c^{2}}{a+b}+\frac{a^{2}}{b+c}+\frac{b^{2}}{c+a} \text{ is the sum in ascending order, } \frac{a^{2}}{a+b}+\frac{b^{2}}{b+c}+\frac{c^{2}}{c+a} \text{ is the sum in random order,}
c2a+b+a2b+c+b2c+aa2a+b+b2b+c+c2c+a,\therefore \frac{c^{2}}{a+b}+\frac{a^{2}}{b+c}+\frac{b^{2}}{c+a} \geq \frac{a^{2}}{a+b}+\frac{b^{2}}{b+c}+\frac{c^{2}}{c+a},

which means c2a2a+b+a2b2b+c+b2c2c+a0\frac{c^{2}-a^{2}}{a+b}+\frac{a^{2}-b^{2}}{b+c}+\frac{b^{2}-c^{2}}{c+a} \geq 0.

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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.