# if $x-y =\sqrt{x}-\sqrt{y}$ with $x\neq y$ then $(1+\frac{1}{x})(1+\frac{1}{y})\geq 25$?

let $$x\neq y$$ be positive real numbers such that :$$x-y= \sqrt{x}-\sqrt{y}$$ , I have tried to prove this inequality $$(1+\frac{1}{x})(1+\frac{1}{y})\geq 25$$ that i have created but i didn't got it.

Attempt I have showed that:$$(\frac{1}{x}+\frac{1}{y})\geq \frac{2}{\sqrt{xy}}$$ using this identity: $$(\sqrt{x}-\sqrt{y})^2\geq0$$ , I also showed that :$$\frac{1}{xy}\geq \frac{1}{16}$$ , Now I have used both result I have got the following inequality :$$(1+\frac{y}{x})(1+\frac{x}{y})\geq 25$$ but not what i have claimed , any way ?

• I am missing something or the inequality is false for $x =y=1$ whike the equality is true? – Tito Eliatron Dec 30 '18 at 22:51
• If $x=y=1$, $x-y=\sqrt x-\sqrt y$, but $(1+{1\over x})(1+{1\over y})=4$, which is not greater than or equal to 25. – Steve Kass Dec 30 '18 at 22:51
• Tkae $\;x=y=1\;$ as an easy counter example.... – DonAntonio Dec 30 '18 at 22:51
• If you omit $x=y$ as solutions, then your condition is equivalent to $\sqrt{x}+\sqrt{y}=1$. – Cheerful Parsnip Dec 30 '18 at 22:52
• @TitoEliatron you mean $\sqrt x+\sqrt y=1$ – Mark Bennet Dec 30 '18 at 23:04

Following the hints of the comments, your condition implies that $$\sqrt{x} + \sqrt{y} = 1$$ AM-GM states $$1 \geq 2\sqrt{\sqrt{xy}} \implies 1/4 \geq \sqrt{xy}$$ Use Cauchy on $$(1+1/x)(1+1/y) \geq (1+1/\sqrt{xy})^2$$ Pluggin in what you got in the AM-GM for $$\sqrt{xy}$$, $$\geq (1+ 4)^2 =25$$

Just to give a different approach, let $$x=1/u^2$$ and $$y=1/v^2$$ with $$u,v\gt0$$. Then

\begin{align} x-\sqrt x=y-\sqrt y &\implies{1\over u^2}-{1\over v^2}={1\over u}-{1\over v}\\ &\implies{(v-u)(v+u)\over u^2v^2}={v-u\over uv}\\ &\implies{v+u\over uv}=1\\ &\implies{1\over u}+{1\over v}=1 \end{align}

provided $$u\not=v$$ (i.e., provided $$x\not=y$$), which allows the cancellation of the $$v-u$$ term. Note this now requires $$u,v\gt1$$. We also have

$${v+u\over uv}=1\implies uv=u+v\implies u^2v^2=(v+u)^2=u^2+v^2+2uv=u^2+v^2+2u+2v$$

It follows that

\begin{align} \left(1+{1\over x}\right)\left(1+{1\over y}\right) &=(1+u^2)(1+v^2)\\ &=1+u^2+v^2+u^2v^2\\ &=1+2u^2+2v^2+2u+2v\\ &={(2u+1)^2+(2v+1)^2\over2}\\ &\gt{(2\cdot1+1)^2+(2\cdot1+1)^2\over2}\\ &=25 \end{align}

By AM-GM $$\left(1+\frac{1}{x}\right)\left(1+\frac{1}{y}\right)=\left(1+\frac{4}{4x}\right)\left(1+\frac{4}{4y}\right)\geq$$ $$\geq\frac{5}{\sqrt[5]{(4x)^4}}\cdot\frac{5}{\sqrt[5]{(4y)^4}}=\frac{25}{\sqrt[5]{4^{8}\left(\sqrt{xy}\right)^8}}\geq\frac{5}{\sqrt[5]{4^8\left(\frac{\sqrt{x}+\sqrt{y}}{2}\right)^{16}}}=25.$$

The function $$y\mapsto \log\left(1+\tfrac{1}{y^2}\right)$$ is convex on $$(0,\infty)$$, so Jensen's inequality $$\mathrm{E}\left[\,\log\left(1+\tfrac{1}{Y^2}\right)\right]\geq \log\left(1+\tfrac{1}{\mathrm{E}[Y]^2}\right)$$ holds for any positive random variable $$Y$$. Letting $$Y=\sqrt{X}$$, in particular we have $$\mathrm{E}\left[\,\log\left(1+\tfrac{1}{X}\right)\right]\geq \log\left(1+\tfrac{1}{\mathrm{E}[\sqrt{X}]^2}\right)$$ for any positive random variable $$X$$.

Now, let $$X$$ be a categorical variable taking on the $$N$$ positive values $$x_0,x_1,\ldots,x_{N-1}$$ with equal probability $$\tfrac{1}{N}$$, and suppose that $$\mathrm{E}[\sqrt{X}]=\frac{1}{N}\sum_{i=0}^{N-1}\sqrt{x_i}=\frac{1}{N}\text{.}$$ Then the inequality above is

$$\frac{1}{N}\sum_{i=0}^{N-1}\log\left(1+\tfrac{1}{x_i}\right)\geq \log(1+N^2)\text{;}$$ taking exponentials, we have shown that $$\boxed{\sum_{i=0}^{N-1}\sqrt{x_i}=1\Rightarrow\prod_{i=0}^{N-1}\left(1+\tfrac{1}{x_i}\right)\geq (1+N^2)^N }\text{.}$$ OP's inequality is the case $$N=2$$.