Let a, b, c be positive real numbers such that ab+bc+ca=1. Show that
3(a+b+c)≤bcaa+cabb+abcc.
Solution
Solution 1. By Holder Inequality we have (cyc∑bcaa)(cyc∑bc)(cyc∑1)≥(cyc∑a)3⇒cyc∑bcaa≥31(cyc∑a)3. So it suffices to have 31(cyc∑a)3≥3(cyc∑a)⇔(cyc∑a)2≥33⇔cyc∑a≥427 Now suppose that ∑cyca<427. It suffices to prove that ∑cycbcaa≥3427, but we have ∑cycbcaa≥33∏cycbcaa=3(abc)−61 (*) by AM-GM inequality 1⇒cyc∑bcaa=cyc∑ab≥3(abc)32⇒(abc)−32≥3⇒(abc)−61≥43≥3(abc)−61≥343=345=321⋅343=3427.
□ Solution 2. First by Cauchy-Schwarz inequality we have (cyc∑bcaa2)(cyc∑bca)≥(cyc∑a)2. So it suffices to prove that (cyc∑a)2cyc∑ab≥3×abc(cyc∑bc)(cyc∑a). Since ∑cycab=1. But by AM-GM and Cauchy-Schwarz we have cyc∑ab≥31cyc∑ab(Cauchy-Schwarz)(1) cyc∑a≥31(cyc∑a)2(Cauchy-Schwarz)(2)
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