The minimum value of x y 2 z ( x 4 + 1 ) ( y 4 + 1 ) ( z 4 + 1 ) as x , y , and z range over the positive reals is equal to C A B , where A and C are coprime and B is squarefree. What is A + B + C ?
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Thanks to @Nathan Ramesh for giving me the crucial inspiration for this problem (although I think he is still stuck on it at this moment, lol ⌣ ¨ ).
Before, for some reason, I was expanding and trying to mega AM-GM bash when I knew all along that it wouldn't work because the degrees weren't right.
It's slightly easier to see and approach it as in @Jubayer Nirjhor 's solution.
Variable separable inequalities generally should be compartmentalized.
Note that by AM-GM inequality, y 4 + 1 ≥ 2 y 2 so we can cancel the y 2 . This trick doesn't work for others. So we split up the 1 so that we have 4 terms to cancel the 4 th power. x 4 + 1 = x 4 + 3 1 + 3 1 + 3 1 ≥ 4 4 3 3 x 4 = 4 3 3 4 x z 4 + 1 = z 4 + 3 1 + 3 1 + 3 1 ≥ 4 4 3 3 z 4 = 4 3 3 4 z
Therefore: x y 2 z ( x 4 + 1 ) ( y 4 + 1 ) ( z 4 + 1 ) ≥ 4 3 3 4 x ⋅ 2 y 2 ⋅ 4 3 3 4 z ⋅ x y 2 z 1 = 9 3 2 3
Thus the answer is 4 4 .
Note: the equality occurs when y 4 = 1 ⟹ y = 1 , x 4 = z 4 = 1 / 3 ⟹ x = z = 1 / 4 3 .
We see that ∂ x ∂ ( x y 2 z ( x 4 + 1 ) ( y 4 + 1 ) ( z 4 + 1 ) ) = 0 for x = 3 1 / 4 and ∂ y ∂ ( x y 2 z ( x 4 + 1 ) ( y 4 + 1 ) ( z 4 + 1 ) ) = 0 for y = 1 . The equation is symmetric in x and z , and the functions are increasing in x , y and z . Hence, the minimum value is 9 3 2 3 . Which gives us the answer 4 4
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We see that x y 2 z ( x 4 + 1 ) ( y 4 + 1 ) ( z 4 + 1 ) = ( x 3 + 3 x 1 + 3 x 1 + 3 x 1 ) ( y 2 + y 2 1 ) ( z 3 + 3 z 1 + 3 z 1 + 3 z 1 )
Using AM-GM on each of these pieces gives ( x 3 + 3 x 1 + 3 x 1 + 3 x 1 ) ( y 2 + y 2 1 ) ( z 3 + 3 z 1 + 3 z 1 + 3 z 1 ) ≥ ( 4 4 2 7 1 ) ( 2 ) ( 4 4 2 7 1 ) = 9 3 2 3
so our answer is 3 2 + 3 + 9 = 4 4 .
Equality case is when ( x , y , z ) = ( 4 3 1 , 1 , 4 3 1 )