Beautiful inequality

Algebra Level 3

Let a a , b b and c c be positive real. Find the minimum value of: c y c a 5 + a 3 + 14 b + c + 2 \large \sum_{cyc}\frac{a^5+a^3+14}{b+c+2}


The answer is 12.

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1 solution

Mohammed Imran
Apr 4, 2020

Let's assume a + b + c = 3 a+b+c=3 . The given expression reduces to c y c a 5 + a 3 + 14 5 a \sum_{cyc} \frac{a^5+a^3+14}{5-a} let f ( x ) = x 5 + x 3 + 14 5 x f(x)=\frac{x^5+x^3+14}{5-x} . Since f ( x ) f(x) is a convex function, by Jensen's Inequality, we have f ( a + b + c 3 ) f ( a ) + f ( b ) + f ( c ) 3 f(\frac{a+b+c}{3}) \leq \frac{f(a)+f(b)+f(c)}{3} so, we have f ( a ) + f ( b ) + f ( c ) 12 f(a)+f(b)+f(c) \geq 12 . And hence the minimum value is 12 \boxed{12}

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