#Min-1

Algebra Level 3

If x x , y y , and z z are real numbers, find the minimum value of cyc x 5 x 3 y z + y z 4 \large \sum_{\text{cyc}} \frac{x^5}{x^3yz+yz^4}


The answer is 1.5.

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1 solution

Multiplying the nominator and denominator of the summand by x x , we have cyc x 5 x 3 y z + y z 4 = cyc x 6 x 4 y z + x y z 4 \displaystyle \sum_{\text{cyc}} \frac {x^5}{x^3yz+yz^4} = \sum_{\text{cyc}} \frac {x^6}{x^4yz+xyz^4} .

Using Titu's lemma :

cyc x 6 x 4 y z + x y z 4 ( x 3 + y 3 + z 3 ) 2 x 4 y z + x y z 4 + x y 4 z + x 4 y z + x y z 4 + x 4 y z ( x 3 + y 3 + z 3 ) 2 2 x y z ( x 3 + y 3 + z 3 ) x 3 + y 3 + z 3 2 x y z Equality occurs when x = y = z 3 2 = 1.5 \begin{aligned} \sum_{\text{cyc}} \frac {x^6}{x^4yz+xyz^4} & \ge \frac {\left(x^3+y^3+z^3\right)^2}{x^4yz+xyz^4+xy^4z+x^4yz+xyz^4 +x^4yz} \\ & \ge \frac {\left(x^3+y^3+z^3\right)^2}{2xyz\left(x^3+y^3+z^3\right)} \\ & \ge \frac {x^3+y^3+z^3}{2xyz} \quad \quad \small \color{#3D99F6} \text{Equality occurs when }x=y=z \\ & \ge \frac 32 = \boxed{1.5} \end{aligned}

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