Minimum value#3

Algebra Level 4

Non-negative real numbers a , b , c a,b,c are such that a + b + c = 2017 a+b+c=2017 . Find the minimum value of

C = 4 ( a 3 + b 3 ) 3 + 4 ( b 3 + c 3 ) 3 + 4 ( c 3 + a 3 ) 3 . C=\sqrt [ 3 ]{ 4\left( { a }^{ 3 }+{ b }^{ 3 } \right) } +\sqrt [ 3 ]{ 4\left( { b }^{ 3 }+{ c }^{ 3 } \right) } +\sqrt [ 3 ]{ 4\left( { c }^{ 3 }+{ a }^{ 3 } \right) } .


The answer is 4034.

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1 solution

Linkin Duck
Apr 1, 2017

From a , b 0 a,b\ge 0 we have ( a b ) 2 ( a + b ) 0 { \left( a-b \right) }^{ 2 }\left( a+b \right) \ge 0

a 3 + b 3 a 2 b + a b 2 ( 1 ) \Longrightarrow { a }^{ 3 }+{ b }^{ 3 }\ge { a }^{ 2 }b+a{ b }^{ 2 }\quad \left( 1 \right)

From ( 1 ) \left( 1 \right) we get 3 ( a 3 + b 3 ) 3 ( a 2 b + a b 2 ) { 3\left( { a }^{ 3 }+{ b }^{ 3 } \right) }\ge { 3\left( { a }^{ 2 }b+a{ b }^{ 2 } \right) } 4 ( a 3 + b 3 ) ( a + b ) 3 \Longrightarrow { 4\left( { a }^{ 3 }+{ b }^{ 3 } \right) }\ge { { \left( a+b \right) }^{ 3 } }

Similarly, 4 ( b 3 + c 3 ) ( b + c ) 3 { 4\left( { b }^{ 3 }+{ c }^{ 3 } \right) }\ge { { \left( b+c \right) }^{ 3 } } and 4 ( c 3 + a 3 ) ( c + a ) 3 { 4\left( { c }^{ 3 }+{ a }^{ 3 } \right) }\ge { { \left( c+a \right) }^{ 3 } }

So, C = 4 ( a 3 + b 3 ) 3 + 4 ( b 3 + c 3 ) 3 + 4 ( c 3 + a 3 ) 3 ( a + b ) + ( b + c ) + ( c + a ) = 2 × 2017 = 4034 C=\sqrt [ 3 ]{ 4\left( { a }^{ 3 }+{ b }^{ 3 } \right) } +\sqrt [ 3 ]{ 4\left( { b }^{ 3 }+{ c }^{ 3 } \right) } +\sqrt [ 3 ]{ 4\left( { c }^{ 3 }+{ a }^{ 3 } \right) } \ge \left( a+b \right) +\left( b+c \right) +\left( c+a \right) =2\times 2017=4034

C m i n = 4034 { C }_{ min }=4034 when a = b = c = 2017 3 a=b=c=\frac { 2017 }{ 3 }

Note: Inequality ( 1 ) \left( 1 \right) is the key.

Note: The inequality 4 ( a 3 + b 3 ) ( a + b ) 3 4 (a^3 + b^3) \geq ( a +b)^3 follows directly from the power mean inequality (qagh) :

a 3 + b 3 2 3 a + b 2 \sqrt[3] { \frac{ a^3 + b^3 } { 2 } } \geq \frac{ a + b } { 2 }

Calvin Lin Staff - 4 years, 2 months ago

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The power mean inequality (qagh) holds in condition that a , b a,b are strictly greater than 0 0 , right? But we can still apply here, it's nice anyway.

Linkin Duck - 4 years, 2 months ago

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Actually, the QAG inequality applies for non-negative reals. This follows from the positive version by just ignoring all the 0 values.

I'm ignoring the HM part, because of 1 0 \frac{1}{0} that would result otherwise.

Calvin Lin Staff - 4 years, 2 months ago

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