Let x , y , z be non-negative real numbers so that x + y + z = 1 . To 2 decimal places, find the minimum value of
( x 3 + y 3 + z 3 ) ( x 2 y 2 + y 2 z 2 + z 2 x 2 ) x y + y z + z x .
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You can get this by u v w as you say, but it's going to be hard--the relevant polynomial has degree 7 so you can't immediately conclude that the minimum occurs at a place where two of the variables are equal or one of the variables is 0 (which it does). I haven't gone through the relevant computations in detail.
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Yup, that's the problem. Degree 7 poly isn't easy at all.
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@Pi Han Goh This is the solution.
Let x = 2 3 3 + 1 , y = 2 3 3 − 1 , z = 0 , we obtain the value of 12. (My most ridiculous part is this one... I mean, how would a random person know that these numbers are optimal???)
Now let's prove it's the minimum.
Let p = x + y + z = 1 , q = x y + y z + z x , r = x y z .
q ≥ 1 2 ( p ( p 2 − 3 q ) + 3 r ) ( q 2 − 2 p r )
= > q ≥ 1 2 ( 1 − 3 q + 3 r ) ( q 2 − 2 r )
Note that p q ≥ 9 r = > 1 2 ( 1 − 3 q + 3 r ) ( q 2 − 2 r ) ≤ 1 2 q 2 ( 1 − 3 q + 3 q ) = 4 q 2 ( 3 = 8 q ) ( x y z ≥ 0 )
It is obviously shown that 4 q ( 3 − 8 q ) ≤ 1 = > ( 1 − 4 q ) ( 1 − 8 q ) ≥ 0 for all q ≥ 4 1 , which leaves us to prove that the inequality above is true for 0 ≤ q ≤ 4 1 .
Back to the original inequality, we can see that q ≥ 1 2 q 2 ( 1 − 3 q ) ≥ 1 2 q 2 ( 1 − 3 q ) + 1 2 r ( 3 q 2 + 6 q − 2 ) − 7 2 r 2 , as 3 q 2 + 6 q − 2 and − 7 2 r 2 are all non-positive.
It remains to prove that q ≥ 1 2 q 2 ( 1 − 3 q ) which is obvious by AM-GM:
4 ∗ 3 q ( 1 − 3 q ) ≤ ( 3 q + 1 − 3 q ) 2 = 1 .
I didn't have much time to write this solution, so if there is anything not-so-easy to understand, just leave a comment here and I'll try to explain.