# Max/Min Problem

For $x$, $y$, and $z$ positive real numbers, find $\frac{z}{x}$ such that $(x,y,z)$ achieves the maximum value of $$\sqrt{\frac{3x+4y}{6x+5y+4z}} + \sqrt{\frac{y+2z}{6x+5y+4z}} + \sqrt{\frac{2z+3x}{6x+5y+4z}}.$$ I found that the maximum value was $\sqrt3$ from Cauchy's Inequality, but I don't know how to actually achieve the maximum. I know that the terms must be proportional, but I don't know how to go from there. Any help on how to proceed would be greatly appreciated, thanks!

• Why don't you post your working, then it would be easier to pinpoint how to get it. – Macavity Nov 11 '14 at 17:21

## 2 Answers

Here's how I would go about it: As the expression is homogeneous, we can set $$6x+5y+4z=1$$ WLOG. Then CS inequality gives: $$(\overline{3x+4y}+\overline{y+2z}+\overline{2z+3x})(1+1+1)\ge \left(\sqrt{3x+4y}+\sqrt{y+2z}+\sqrt{2z+3x} \right)^2$$

So we get $$\sqrt{3x+4y}+\sqrt{y+2z}+\sqrt{2z+3x} \le \sqrt3$$ and equality is possible iff $$\dfrac{3x+4y}1=\dfrac{y+2z}1=\dfrac{2z+3x}1 \implies x:y:z = 1:3:6$$

• Shouldn't it be $\sqrt{1+1+1}$ instead of $1+1+1$? – YuiTo Cheng Apr 5 at 15:47
• @YuiToCheng It is correct as written. – Macavity Apr 6 at 1:15

It's $(a+b+c)^2\leq3(a^2+b^2+c^2)$ The equality occurs for $3x+4y=y+2z=2z+3x$.