Let a, b and c be positive real numbers. Prove that b+ca+b+c+a+c+aa+b+c+b+a+ba+b+c+c≥2a+b+c9+33
Solution
The inequality is homogeneous, hence without loss of generality, we can assume that a+b+c=1. The given inequality transforms into A1−a1+1−b1+1−c1+B1−aa+1−bb+1−cc≥29+33. By the inequality of arithmetic and harmonic means, we have A≥3−(a+b+c)9=29. On the other hand, by the Cauchy-Bunyakovsky-Schwarz inequality, we have B⋅(aa(1−a)+bb(1−b)+cc(1−c))≥1, and it suffices to show that aa(1−a)+bb(1−b)+cc(1−c)≤332. By the inequality of geometric and arithmetic means, we have 4a(1−a)2≤(3a+21−a+21−a)3=271, i.e. a(1−a)≤332, and analogously b(1−b)≤332, c(1−c)≤332. Finally, aa(1−a)+bb(1−b)+cc(1−c)≤(a+b+c)⋅332=332, and the given inequality is proved.
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