Cool Inequality #9 - 2 days to 2016, complex one

Algebra Level 5

a b + c + b c + a \large{\frac{a}{b+c}+\frac{b}{c+a}}

If a , b , c a,b,c are positive reals with a b c a \ge b \ge c and 2 b b + c + a c + 2 c a + c = 17 \dfrac{2b}{b+c} +\dfrac{a}{c} + \dfrac{2c}{a+c} =17 . Find the maximum value of the expression above.


The answer is 8.

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1 solution

Department 8
Dec 30, 2015

We have

a b c a + c b + c 2 c a + c b + c 1 b + c 1 a + c a + c 2 c 1 2 c 1 a + c b + c 2 c 1 b + c 1 2 c \Large{a\ge b\ge c\\ a+c\ge b+c\ge 2c\\ \\ \Longrightarrow a+c\ge b+c\\ \frac { 1 }{ b+c } \ge \frac { 1 }{ a+c } \\ \\ \Longrightarrow a+c\ge 2c\\ \frac { 1 }{ 2c } \ge \frac { 1 }{ a+c } \\ \\ \Longrightarrow b+c\ge 2c\\ \frac { 1 }{ b+c } \ge \frac { 1 }{ 2c } }

From these have two sets

a b c 1 2 c 1 b + c 1 c + a \Large{a\ge b\ge c \\ \frac { 1 }{ 2c } \ge \frac { 1 }{ b+c } \ge \frac { 1 }{ c+a } }

Using Rearrangement Inequality,

a 2 c + b b + c + c c + a a b + c + b c + a + c 2 c 17 2 1 2 a b + c + b c + a 8 a b + c + b c + a \Large{\frac { a }{ 2c } +\frac { b }{ b+c } +\frac { c }{ c+a } \ge \frac { a }{ b+c } +\frac { b }{ c+a } +\frac { c }{ 2c } \\ \frac { 17 }{ 2 } -\frac { 1 }{ 2 } \ge \frac { a }{ b+c } +\frac { b }{ c+a } \\ \\ 8\ge \frac { a }{ b+c } +\frac { b }{ c+a } }

Hence the maximum value is 8 \boxed{8}

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