Non-negative real numbers a , b , c are such that a + b + c = 2 0 1 7 . Find the minimum value of
C = 3 4 ( a 3 + b 3 ) + 3 4 ( b 3 + c 3 ) + 3 4 ( c 3 + a 3 ) .
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Note: The inequality 4 ( a 3 + b 3 ) ≥ ( a + b ) 3 follows directly from the power mean inequality (qagh) :
3 2 a 3 + b 3 ≥ 2 a + b
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The power mean inequality (qagh) holds in condition that a , b are strictly greater than 0 , right? But we can still apply here, it's nice anyway.
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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 0 1 that would result otherwise.
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From a , b ≥ 0 we have ( a − b ) 2 ( a + b ) ≥ 0
⟹ a 3 + b 3 ≥ a 2 b + a b 2 ( 1 )
From ( 1 ) we get 3 ( a 3 + b 3 ) ≥ 3 ( a 2 b + a b 2 ) ⟹ 4 ( a 3 + b 3 ) ≥ ( a + b ) 3
Similarly, 4 ( b 3 + c 3 ) ≥ ( b + c ) 3 and 4 ( c 3 + a 3 ) ≥ ( c + a ) 3
So, C = 3 4 ( a 3 + b 3 ) + 3 4 ( b 3 + c 3 ) + 3 4 ( c 3 + a 3 ) ≥ ( a + b ) + ( b + c ) + ( c + a ) = 2 × 2 0 1 7 = 4 0 3 4
C m i n = 4 0 3 4 when a = b = c = 3 2 0 1 7
Note: Inequality ( 1 ) is the key.