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Algebra Difficulty 7.5 National olympiad, round 2 Prove it

35. Prove that for any positive real numbers a,b,ca, b, c, we have 1<aa2+b2+bb2+c2+cc2+a2322(20041<\frac{a}{\sqrt{a^{2}+b^{2}}}+\frac{b}{\sqrt{b^{2}+c^{2}}}+\frac{c}{\sqrt{c^{2}+a^{2}}} \leqslant \frac{3 \sqrt{2}}{2} \cdot(2004 China Western Mathematical Olympiad Problem)

Solution

35. The left side of the inequality is easy to prove:
aa2+b2+bb2+c2+cc2+a2>aa+b+c+ba+b+c+ca+b+c=1\frac{a}{\sqrt{a^{2}+b^{2}}}+\frac{b}{\sqrt{b^{2}+c^{2}}}+\frac{c}{\sqrt{c^{2}+a^{2}}}>\frac{a}{a+b+c}+\frac{b}{a+b+c}+\frac{c}{a+b+c}=1

Now we prove the right side.
By the Cauchy-Schwarz inequality, we have
(aa2+b2+bb2+c2+cc2+a2)2=[a2+c2a(a2+b2)(a2+c2)+b2+a2b(b2+c2)(b2+a2)+c2+b2c(c2+a2)(c2+b2)]2[(a2+c2)+(b2+a2)+(c2+b2)][a2(a2+b2)(a2+c2)+b2(b2+c2)(b2+a2)+c2(c2+a2)(c2+b2)]=2(a2+b2+c2)[a2(a2+b2)(a2+c2)+b2(b2+c2)(b2+a2)+c2(c2+a2)(c2+b2)]\begin{array}{l} \left(\frac{a}{\sqrt{a^{2}+b^{2}}}+\frac{b}{\sqrt{b^{2}+c^{2}}}+\frac{c}{\sqrt{c^{2}+a^{2}}}\right)^{2}= \\ {\left[\sqrt{a^{2}+c^{2}} \cdot \frac{a}{\sqrt{\left(a^{2}+b^{2}\right)\left(a^{2}+c^{2}\right)}}+\sqrt{b^{2}+a^{2}} \cdot \frac{b}{\sqrt{\left(b^{2}+c^{2}\right)\left(b^{2}+a^{2}\right)}}+\right.} \\ \left.\sqrt{c^{2}+b^{2}} \cdot \frac{c}{\sqrt{\left(c^{2}+a^{2}\right)\left(c^{2}+b^{2}\right)}}\right]^{2} \leqslant \\ {\left[\left(a^{2}+c^{2}\right)+\left(b^{2}+a^{2}\right)+\left(c^{2}+b^{2}\right)\right] \cdot} \\ {\left[\frac{a^{2}}{\left(a^{2}+b^{2}\right)\left(a^{2}+c^{2}\right)}+\frac{b^{2}}{\left(b^{2}+c^{2}\right)\left(b^{2}+a^{2}\right)}+\frac{c^{2}}{\left(c^{2}+a^{2}\right)\left(c^{2}+b^{2}\right)}\right]=} \\ 2\left(a^{2}+b^{2}+c^{2}\right)\left[\frac{a^{2}}{\left(a^{2}+b^{2}\right)\left(a^{2}+c^{2}\right)}+\right. \\ \left.\frac{b^{2}}{\left(b^{2}+c^{2}\right)\left(b^{2}+a^{2}\right)}+\frac{c^{2}}{\left(c^{2}+a^{2}\right)\left(c^{2}+b^{2}\right)}\right] \end{array}

Now we prove
a2(a2+b2+c2)(a2+b2)(a2+c2)+b2(a2+b2+c2)(b2+c2)(b2+a2)+c2(a2+b2+c2)(c2+a2)(c2+b2)98(1)8(a2+b2+c2)(a2b2+b2c2+c2a2)9(a2+b2)(b2+c2)(c2+\begin{array}{l} \frac{a^{2}\left(a^{2}+b^{2}+c^{2}\right)}{\left(a^{2}+b^{2}\right)\left(a^{2}+c^{2}\right)}+\frac{b^{2}\left(a^{2}+b^{2}+c^{2}\right)}{\left(b^{2}+c^{2}\right)\left(b^{2}+a^{2}\right)}+\frac{c^{2}\left(a^{2}+b^{2}+c^{2}\right)}{\left(c^{2}+a^{2}\right)\left(c^{2}+b^{2}\right)} \leqslant \frac{9}{8} \\ (1) \Leftrightarrow 8\left(a^{2}+b^{2}+c^{2}\right)\left(a^{2} b^{2}+b^{2} c^{2}+c^{2} a^{2}\right) \leqslant 9\left(a^{2}+b^{2}\right)\left(b^{2}+c^{2}\right)\left(c^{2}+\right. \end{array}
a2)a4b2+b4c2+c4a2+a2b4+b2c4+c2a46a2b2c2\left.a^{2}\right) \Leftrightarrow a^{4} b^{2}+b^{4} c^{2}+c^{4} a^{2}+a^{2} b^{4}+b^{2} c^{4}+c^{2} a^{4} \geqslant 6 a^{2} b^{2} c^{2}. This follows from the AM-GM inequality.
Thus, 1<aa2+b2+bb2+c2+cc2+a23221<\frac{a}{\sqrt{a^{2}+b^{2}}}+\frac{b}{\sqrt{b^{2}+c^{2}}}+\frac{c}{\sqrt{c^{2}+a^{2}}} \leqslant \frac{3 \sqrt{2}}{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.