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Algebra Difficulty 7.2 National olympiad, round 2 Prove it

38. Let a,b,ca, b, c be non-negative real numbers, prove:
a2+2bcb2+c2+b2+2cac2+a2+c2+2aba2+b23\sqrt{\frac{a^{2}+2 b c}{b^{2}+c^{2}}}+\sqrt{\frac{b^{2}+2 c a}{c^{2}+a^{2}}}+\sqrt{\frac{c^{2}+2 a b}{a^{2}+b^{2}}} \geq 3 (Vo Quoc Ba Can, Vu Dinh Quy)

Solution

To prove: Without loss of generality, assume abca \geq b \geq c. First, we will prove
a2+c2b2+c2+b2+c2c2+a2ab+ba\sqrt{\frac{a^{2}+c^{2}}{b^{2}+c^{2}}}+\sqrt{\frac{b^{2}+c^{2}}{c^{2}+a^{2}}} \geq \sqrt{\frac{a}{b}}+\sqrt{\frac{b}{a}}

In fact, this inequality is equivalent to
a2+c2b2+c2+b2+c2c2+a2ab+ba(ab)2(a+b)(abc)2ab(c2+a2)(b2+c2)0\frac{a^{2}+c^{2}}{b^{2}+c^{2}}+\frac{b^{2}+c^{2}}{c^{2}+a^{2}} \geq \frac{a}{b}+\frac{b}{a} \Leftrightarrow \frac{(a-b)^{2}(a+b)(a b-c)^{2}}{a b\left(c^{2}+a^{2}\right)\left(b^{2}+c^{2}\right)} \geq 0

This is true because abca \geq b \geq c. Using this result, we have
a2+2bcb2+c2+b2+2cac2+a2+c2+2aba2+b2c2+a2b2+c2+b2+c2c2+a2+2aba2+b2ab+ba+2aba2+b2\begin{array}{l} \sqrt{\frac{a^{2}+2 b c}{b^{2}+c^{2}}}+\sqrt{\frac{b^{2}+2 c a}{c^{2}+a^{2}}}+\sqrt{\frac{c^{2}+2 a b}{a^{2}+b^{2}}} \geq \sqrt{\frac{c^{2}+a^{2}}{b^{2}+c^{2}}}+\sqrt{\frac{b^{2}+c^{2}}{c^{2}+a^{2}}}+\sqrt{\frac{2 a b}{a^{2}+b^{2}}} \\ \geq \sqrt{\frac{a}{b}}+\sqrt{\frac{b}{a}}+\sqrt{\frac{2 a b}{a^{2}+b^{2}}} \end{array}

Let x=ab+ba2x=\sqrt{\frac{a}{b}}+\sqrt{\frac{b}{a}} \geq 2. If x3x \geq 3, the inequality is obviously true. Otherwise, assume x3x \leq 3, we only need to prove
ab+ba+2aba2+b2=x+2x223\sqrt{\frac{a}{b}}+\sqrt{\frac{b}{a}}+\sqrt{\frac{2 a b}{a^{2}+b^{2}}}=x+\sqrt{\frac{2}{x^{2}-2}} \geq 3

Since
2x22(3x)2=(x2)2(x2+2x+5)x230\frac{2}{x^{2}-2}-(3-x)^{2}=\frac{(x-2)^{2}\left(-x^{2}+2 x+5\right)}{x^{2}-3} \geq 0

Equality holds if and only if a=b,c=0a=b, c=0 and their permutations.

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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.