Good morning,

Assuming that $x$,$y$,$z$ are positive real numbers and $x \le y \le z$ and $ x \cdot y \cdot z = 1$.

How can I prove that $(x+1) \cdot (z+1) > 3$?

I know that $x$ has to be less than $1$ and $z$ greater than $1$ and I have already tried to rearrange that term but this didn't help me.

So do you have any ideas how to prove that?


Let $y < z$

Set $y_1 = z_1 = \sqrt{yz}$

$(x+1)(z+1)> (x+1)(z_1+1)$

So, you must prove you task only for $z = y$ => $z=\frac{1}{\sqrt{x}}$, $x \le 1$

$(x+1)(z+1) = 1 + x + \sqrt{x} + \frac{1}{\sqrt{x}} \ge 3+x$
(by $t + \frac{1}{t} \ge 2$ )


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