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Algebra Difficulty 6.0 AIME, harder Prove it Croatia

Let aa, bb and cc be positive real numbers. Prove that
a+b+c+ab+c+a+b+c+bc+a+a+b+c+ca+b9+332a+b+c \frac{\sqrt{a+b+c}+\sqrt{a}}{b+c} + \frac{\sqrt{a+b+c}+\sqrt{b}}{c+a} + \frac{\sqrt{a+b+c}+\sqrt{c}}{a+b} \ge \frac{9+3\sqrt{3}}{2\sqrt{a+b+c}}

Solution

The inequality is homogeneous, hence without loss of generality, we can assume that a+b+c=1a+b+c=1. The given inequality transforms into
11a+11b+11cA+a1a+b1b+c1cB9+332. \underbrace{\frac{1}{1-a} + \frac{1}{1-b} + \frac{1}{1-c}}_{A} + \underbrace{\frac{\sqrt{a}}{1-a} + \frac{\sqrt{b}}{1-b} + \frac{\sqrt{c}}{1-c}}_{B} \ge \frac{9+3\sqrt{3}}{2}.
By the inequality of arithmetic and harmonic means, we have
A93(a+b+c)=92. A \ge \frac{9}{3 - (a + b + c)} = \frac{9}{2}.
On the other hand, by the Cauchy-Bunyakovsky-Schwarz inequality, we have
B(aa(1a)+bb(1b)+cc(1c))1, B \cdot \left( a\sqrt{a}(1-a) + b\sqrt{b}(1-b) + c\sqrt{c}(1-c) \right) \ge 1,
and it suffices to show that
aa(1a)+bb(1b)+cc(1c)233. a\sqrt{a}(1-a) + b\sqrt{b}(1-b) + c\sqrt{c}(1-c) \le \frac{2}{3\sqrt{3}}.
By the inequality of geometric and arithmetic means, we have
a(1a)24(a+1a2+1a23)3=127, \frac{a(1-a)^2}{4} \le \left( \frac{a + \frac{1-a}{2} + \frac{1-a}{2}}{3} \right)^3 = \frac{1}{27},
i.e. a(1a)233\sqrt{a}(1-a) \le \frac{2}{3\sqrt{3}}, and analogously b(1b)233\sqrt{b}(1-b) \le \frac{2}{3\sqrt{3}}, c(1c)233\sqrt{c}(1-c) \le \frac{2}{3\sqrt{3}}.
Finally,
aa(1a)+bb(1b)+cc(1c)(a+b+c)233=233, a\sqrt{a}(1-a) + b\sqrt{b}(1-b) + c\sqrt{c}(1-c) \le (a+b+c) \cdot \frac{2}{3\sqrt{3}} = \frac{2}{3\sqrt{3}},
and the given inequality is proved.

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