Seems like u v w uvw

Algebra Level 5

Let x , y , z x,y,z be non-negative real numbers so that x + y + z = 1 x+y+z=1 . To 2 decimal places, find the minimum value of

x y + y z + z x ( x 3 + y 3 + z 3 ) ( x 2 y 2 + y 2 z 2 + z 2 x 2 ) . \frac{xy+yz+zx} {({ x }^{ 3 }+{ y }^{ 3 }+{ z }^{ 3 })({ x }^{ 2 }{ y }^{ 2 }+{ y }^{ 2 }{ z }^{ 2 }+{ z }^{ 2 }{ x }^{ 2 }) } .


The answer is 12.00.

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1 solution

Steven Jim
Sep 6, 2017

@Pi Han Goh This is the solution.

Let x = 3 + 1 2 3 , y = 3 1 2 3 , z = 0 x=\frac { \sqrt { 3 } +1 }{ 2\sqrt { 3 } } ,y=\frac { \sqrt { 3 } -1 }{ 2\sqrt { 3 } } , 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 p=x+y+z=1, q=xy+yz+zx, r=xyz .

q 12 ( p ( p 2 3 q ) + 3 r ) ( q 2 2 p r ) q\ge 12(p({ p }^{ 2 }-3q)+3r)({ q }^{ 2 }-2pr)

= > q 12 ( 1 3 q + 3 r ) ( q 2 2 r ) =>q\ge 12(1-3q+3r)({ q }^{ 2 }-2r)

Note that p q 9 r pq \ge 9r = > 12 ( 1 3 q + 3 r ) ( q 2 2 r ) 12 q 2 ( 1 3 q + q 3 ) = 4 q 2 ( 3 = 8 q ) =>12(1-3q+3r)({ q }^{ 2 }-2r) \le 12{ q }^{ 2 }(1-3q+\frac { { q } }{ 3 } )=4{ q }^{ 2 }(3=8q) ( x y z 0 xyz \ge 0 )

It is obviously shown that 4 q ( 3 8 q ) 1 = > ( 1 4 q ) ( 1 8 q ) 0 4q(3-8q) \le 1 => (1-4q)(1-8q) \ge 0 for all q 1 4 q \ge \frac { 1 }{ 4 } , which leaves us to prove that the inequality above is true for 0 q 1 4 0 \le q \le \frac { 1 }{ 4 } .

Back to the original inequality, we can see that q 12 q 2 ( 1 3 q ) 12 q 2 ( 1 3 q ) + 12 r ( 3 q 2 + 6 q 2 ) 72 r 2 q\ge 12{ q }^{ 2 }(1-3q)\ge 12{ q }^{ 2 }(1-3q)+12r(3{ q }^{ 2 }+6q-2)-72{ r }^{ 2 } , as 3 q 2 + 6 q 2 3{ q }^{ 2 }+6q-2 and 72 r 2 -72r^2 are all non-positive.

It remains to prove that q 12 q 2 ( 1 3 q ) q\ge 12{ q }^{ 2 }(1-3q) which is obvious by AM-GM:

4 3 q ( 1 3 q ) ( 3 q + 1 3 q ) 2 = 1 4*3q(1-3q) \le (3q+1-3q)^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.

You can get this by u v w uvw as you say, but it's going to be hard--the relevant polynomial has degree 7 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.

Patrick Corn - 3 years, 5 months ago

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Yup, that's the problem. Degree 7 poly isn't easy at all.

Steven Jim - 3 years, 5 months ago

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