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Algebra Difficulty 3.8 AMC 10/12 Prove it North Macedonia

Let aa, bb and cc be positive real numbers such that abc=1abc=1. Prove that the following inequality holds
12(a+b+c)+11+a+11+b+11+c3 \frac{1}{2}(\sqrt{a} + \sqrt{b} + \sqrt{c}) + \frac{1}{1+a} + \frac{1}{1+b} + \frac{1}{1+c} \ge 3
When does equality hold?

Solution

Since (1bc)20(1-\sqrt{bc})^2 \ge 0 it follows that 1+bc2bc1+bc \ge 2\sqrt{bc}, i.e. 12bc11+bc\frac{1}{2\sqrt{bc}} \ge \frac{1}{1+bc}. We get that
a2+11+a=12bc+11+a11+bc+11+a=11+1a+11+a=1(1). \frac{\sqrt{a}}{2} + \frac{1}{1+a} = \frac{1}{2\sqrt{bc}} + \frac{1}{1+a} \ge \frac{1}{1+bc} + \frac{1}{1+a} = \frac{1}{1+\frac{1}{a}} + \frac{1}{1+a} = 1\ldots(1).
In the same way we prove that
b2+11+b1(2) \frac{\sqrt{b}}{2} + \frac{1}{1+b} \ge 1\ldots(2)
and
c2+11+c1(3). \frac{\sqrt{c}}{2} + \frac{1}{1+c} \ge 1\ldots(3).

By adding (1), (2) and (3) we get the required inequality. Let us note that equality holds if and only if 1=bc1=\sqrt{bc}, i.e. a=1a=1. In the same way we get b=1b=1 and c=1c=1.

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