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Prove continuity of $f: [\frac{1}{2},\infty ) \rightarrow \mathbb{R}: x \mapsto \sqrt{2x-1}$ for $x_0>\frac{1}{2}$ with Epsilon-delta definition of continuity

show:
$\forall \epsilon >0 \ \exists \delta>0 \ \forall x \in [\frac{1}{2}, \infty): ( \ |x-x_0|<\delta \Rightarrow |f(x)-f(x_0)|<\epsilon \ )$

$|x-x_0|<\delta$

$|\sqrt{2x-1} - \sqrt{2x_0-1} | = |\frac{(2x-1)-(2x_0-1)}{\sqrt{2x-1} + \sqrt{2x_0-1}}| = |\frac{2(x-x_0)}{\sqrt{2x-1} + \sqrt{2x_0-1}}|=\frac{2|x-x_0|}{\sqrt{2x-1} + \sqrt{2x_0-1}}$

$\sqrt{2x-1}>0 \Rightarrow \frac{2|x-x_0|}{\sqrt{2x-1} + \sqrt{2x_0-1}}<\frac{2|x-x_0|}{\sqrt{2x_0-1}} \Rightarrow |f(x)-f(x_0)|<\frac{2\delta}{\sqrt{2x_0-1}}$

let $\delta=\frac{\epsilon \sqrt{2x_0-1}}{2} \Rightarrow |f(x)-f(x_0)|<\frac{\epsilon \sqrt{2x_0-1}}{\sqrt{2x_0-1}}=\epsilon$

Which shows $\forall \epsilon >0 \ \exists \delta>0 \ \forall x \in [\frac{1}{2}, \infty): ( \ |x-x_0|<\delta \Rightarrow |f(x)-f(x_0)|<\epsilon \ )$
and therefore the continuity of $f(x)$ for $x_0>\frac{1}{2}$ is proven.

Is this prove correct?

Edit: changed $x=\frac{1}{2}$ to $x_0>\frac{1}{2}$

jaki
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    Can you just use $x_0=\frac{1}{2}$ which will reduce the calculation? You overkilled it. – Hypernova Dec 06 '22 at 09:06
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    Also it doesn’t work for $x_0=\frac{1}{2}$ since $\delta=0$ – Hypernova Dec 06 '22 at 09:09
  • true that. sorry my fault, but for $ x>\frac{1}{2}$ it should be correct?!? – jaki Dec 06 '22 at 09:14
  • To my eyes it seems correct. For $x_0=\frac{1}{2}$ do the same thing then the $\sqrt{2x_0-1}$ term will vanish and you'll get it. – Hypernova Dec 06 '22 at 09:20
  • Um... so which is it? Do you have to prove $f$ is continuos and $x =\frac 12$? or for all $x$ so that $x > \frac 12$? – fleablood Dec 06 '22 at 09:37
  • it is for all x so that $x>\frac{1}{2}$ – jaki Dec 06 '22 at 09:44
  • Oh, you are correct. I was confusing $x_0$ with the $x$ you were choosing in the interval rather than with the point at which you are evaluating if $f$ is continuous. I think your proof is good. – fleablood Dec 06 '22 at 09:48
  • Going forward with your future postings, please try to make more of an effort to proofread your posting. It isn't nice to impose the Shooting Gallery Blues on MathSE reviewers, where they have to try to hit a moving target. – user2661923 Dec 06 '22 at 09:50

1 Answers1

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For $x_0=\frac{1}{2}$:

$|f(x)-f(\frac{1}{2})|=|f(x)|=\left|\sqrt{2x-1}\right|<\epsilon$

$\forall x \in ( \frac{1}{2},\frac{1}{2}+\delta): \left|\sqrt{2x-1}\right| \stackrel{!}{<} \epsilon$

choose: $\delta:= \frac{\epsilon^{2}}{2} \Rightarrow \forall x \in ( \frac{1}{2},\frac{1}{2}+\delta): \left|\sqrt{2x-1}\right| < \epsilon$

And therefore the continuity of $f(x)$ in $x_0=\frac{1}{2}$ is proven

jaki
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    It's not correct, $(2x-1)/\sqrt{2x-1} < 2x - 1$ is true only if $\sqrt{2x-1} > 1$. – Ennar Dec 06 '22 at 09:52
  • As I said, it's incorrect, jaki. Also, this is not an answer to your question since you changed the question for no good reason. I'm not sure why are you so keen on overcomplicating stuff while you simply have to solve inequality $\sqrt{2x-1} < \varepsilon$. – Ennar Dec 06 '22 at 10:05
  • jaki, by definition it is precisely enough to show that all $x\in (1/2,1/2+\delta)$ satisfy the inequality. So pick $\delta$ so they do. – Ennar Dec 06 '22 at 10:19
  • Changed it again. Hope it is correct now. – jaki Dec 06 '22 at 10:41
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    It is correct now. – Ennar Dec 06 '22 at 14:15