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Dethrone
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I read this somewhere:
Somewhere in the middle, you have $\displaystyle -6<x-3<-4$, which, in my textbook says we can rewrite as $|x-3|<6$. Why does that hold? Doesn't that interval consist of $-6<x-3<6$, so you are actually enlarging the domain? Or is that why we need the notation, $\displaystyle \delta=\min\left(1,\frac{\epsilon}{6}\right)$ to say that even if it does takes on values of $-4 < x < 6$ which were part of the added domain, it must always be less than $1$ too?
For the notation $\delta=\min\left(1,\frac{\epsilon}{6}\right)$ , what does that actually mean? Does it mean that the value of which we are actually restricting $\delta$ should actually be the smaller of the two conditions, whatever it may be?
MarkFL said:I can give you an example I posted as a solution at another forum a couple of years ago:
We are given to prove:
\(\displaystyle \lim_{x\to-2}\left(x^2-x-3\right)=3\)
For any given \(\displaystyle \epsilon>0\) we wish to find a \(\displaystyle \delta\) so that:
\(\displaystyle \left|x^2-x-3-3\right|=\left|x^2-x-6\right|<\epsilon\) whenever \(\displaystyle 0<|x+2|<\delta\)
To do this, consider:
\(\displaystyle \left|x^2-x-6\right|=|x+2||x-3|\)
Thus, to make:
\(\displaystyle |x+2||x-3|<\epsilon\)
we need only make:
\(\displaystyle 0<|x+2|<\frac{\epsilon}{|x-3|}\)
Now, we want to define \(\displaystyle \delta=\frac{\epsilon}{|x-3|}\), but we cannot because \(\displaystyle \delta\) is supposed to be dependent on \(\displaystyle \epsilon\) only. To get around this, we will replace \(\displaystyle |x-3|\) with a number \(\displaystyle M\) which satisfies:
\(\displaystyle |x-3|\le M\)
So now we may write:
\(\displaystyle |x+2|<\frac{\epsilon}{M}\)
and proceed as before, taking \(\displaystyle \delta=\frac{\epsilon}{M}\). But, we have a problem, as there is no number \(\displaystyle M\) that satisfies:
\(\displaystyle |x-3|\le M\) for all real numbers $x$. However, we are only interested in those close to \(\displaystyle x=1\). It doesn't matter how close, we just want to bound \(\displaystyle |x-3|\) by restricting $x$ near 1, and any restriction will do. For example, if we require:
\(\displaystyle 0<|x+2|<1\), i.e., $x$ should be less than 1 unit away from 1, or equivalently \(\displaystyle \delta=1\), then we have:
\(\displaystyle -1<x+2<1\)
\(\displaystyle -6<x-3<-4\)
We can now take \(\displaystyle M=6\), so we should let \(\displaystyle \delta=\frac{\epsilon}{6}\). Remember that we also need:
\(\displaystyle |x+2|<1\) so that we define \(\displaystyle \delta=\min\left(1,\frac{\epsilon}{6}\right)\), then:
\(\displaystyle 0<|x+2|<\delta\) implies \(\displaystyle |x+2|<1\) and \(\displaystyle |x+2|<\frac{\epsilon}{6}\). We can now write the proof.
Let \(\displaystyle \epsilon>0\) and define \(\displaystyle \delta=\min\left(1,\frac{\epsilon}{6}\right)\). Then if \(\displaystyle 0<|x+2|<\delta\), we have:
\(\displaystyle \left|x^2-x-6\right|=|x+2||x-3|\)
\(\displaystyle \left|x^2-x-6\right|<6|x+2|\) since \(\displaystyle |x+2|<\delta\) and \(\displaystyle \delta\le1\)
\(\displaystyle \left|x^2-x-6\right|<6\left(\frac{\epsilon}{6}\right)=\epsilon\) since \(\displaystyle |x+2|<\delta\) and \(\displaystyle \delta\le\frac{\epsilon}{6}\)
Therefore, we have shown that \(\displaystyle 0<|x+2|<\delta\) implies \(\displaystyle \left|x^2-x-6\right|<\epsilon\) which shows
\(\displaystyle \lim_{x\to1}\left(x^2-x-3\right)=3\) by definition.
Somewhere in the middle, you have $\displaystyle -6<x-3<-4$, which, in my textbook says we can rewrite as $|x-3|<6$. Why does that hold? Doesn't that interval consist of $-6<x-3<6$, so you are actually enlarging the domain? Or is that why we need the notation, $\displaystyle \delta=\min\left(1,\frac{\epsilon}{6}\right)$ to say that even if it does takes on values of $-4 < x < 6$ which were part of the added domain, it must always be less than $1$ too?
For the notation $\delta=\min\left(1,\frac{\epsilon}{6}\right)$ , what does that actually mean? Does it mean that the value of which we are actually restricting $\delta$ should actually be the smaller of the two conditions, whatever it may be?