# Working with the (ε,δ) definition of limits to find the δ of

I'm introducing myself to the $(\varepsilon, \delta)$ definition of limits, and I'm encountering a few issues.

When proving the $\lim_{x \to c}f(x) = L$ $$\forall \varepsilon > 0, \ \exists \delta = \delta(\varepsilon) > 0 : 0 < |x - c| < \delta \implies |f(x) - L| < \varepsilon$$

When considering $\lim_{x \to 2}(2x - 5) = -1$

Let $\forall \varepsilon > 0$

Choose $\delta = \dfrac{\varepsilon}{2}$

Assume $0 < |x - 2| < \delta$

Then,

$$|2x - 5 - (-1)| < \varepsilon,$$

$$\\|2x - 4| < \varepsilon \\2|x - 2| < \varepsilon \\|x - 2| < \delta \\2|x - 2| < 2\delta \\ \therefore \delta = \dfrac{\varepsilon}{2}$$

Forgive my mistakes, I'm still quite new to this. I believe that my proof is mostly accurate (do correct me if I'm wrong, please).

My biggest issue comes with solving this other problem:

I am to suppose $|f(x)-7| < 0.2$ whenever $0 < x < 7$.

Find all values of $\delta > 0$ such that $|f(x) - 7| < 0.2$ whenever $0 < |x-2| < \delta$.

I've not got a good idea of how to approach this with an arbitrary $f(x)$.

• But what is $f(x)$ ? is it the same as before, i.e. $(2x-5)$ ? If so, it's limit is $7$ for $x=6$; thus, it does not work with $|x-2|$ ... – Mauro ALLEGRANZA Feb 11 '15 at 12:01
• @MauroALLEGRANZA The two are completely separate, sorry. I'm trying to determine the second independent from the first. – user146046 Feb 11 '15 at 12:04
• That's what I'm confused about, actually. I don't have an f(x). I don't know how to find \delta without knowing f(x). – user146046 Feb 11 '15 at 12:09
• For the second question: What is the biggest number $\delta$ you can choose such that if x satisfies the inequality $0 < |x-2| < \delta$ it is guaranteed to satisfy $0 < x < 7$? – user159517 Feb 11 '15 at 12:09
• @user159517 Looking at that, it seems that 5 would be the value. However, the value is 2, apparently. I don't know how. – user146046 Feb 11 '15 at 12:14

We want to find all $\delta > 0$ such that $0 < |x-2| < \delta$ implies $0 < x < 7$. We distinguish two cases:
1. $x \geq 2$: Then we have $0 < x - 2 < \delta \; \Leftrightarrow \; 2 < x < \delta + 2$. From the condition $x < 7$ we see that $\delta \leq 5$ must be satisfied.
2. $x < 2$: Then we get $0 < 2 - x < \delta \; \Leftrightarrow \; -2 < -x < \delta - 2 \; \Leftrightarrow \; 2 - \delta < x < 2$. For $\delta \leq 2$ the condition $0 < x$ is satisfied.
So we have two conditions on $\delta$ : $\delta \leq 2$ and $\delta \leq 5$: Clearly, we can reduce this to $\delta \leq 2$. Hence the set of all $\delta > 0$ such that $0 < |x-2| < \delta$ implies $0 < x < 7$ is exactly the set $(0,2]$.