# prove limit by definition

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• August 9th 2011, 01:59 AM
transgalactic
prove limit by definition
prove that $lim_{x\rightarrow2}\frac{x}{x+1}\neq1$ by epsilon delta
defintion.
we need to prove that their is $\varepsilon>0$ so for every $\delta>0$
their is x $|x-2|<\delta$ and $|\frac{x}{x+1}-1|\geq\varepsilon$
so we are free to choose our epsilon and our x
i choose $\varepsilon=2$
x cannot be equal to -1 because of the denominator.
and we need to choose x for which $|\frac{x}{x+1}-1|\geq2$
$|\frac{x-x-1}{x+1}|\geq2$ -> $|\frac{1}{x+1}|\geq2$ -> $\frac{1}{2}\geq|x+1|$
-> $-\frac{1}{2}\leq x+1\leq\frac{1}{2}$ -> $-\frac{3}{2}\leq x\leq-\frac{1}{2}$
but x=-1 there so we have a problem.
where is the problem in my way???

the prof solved it by $\varepsilon=1/6$
$2>x>max\{2-\delta/2,1\}$
cant understand why he choose such x
• August 9th 2011, 02:22 AM
CaptainBlack
Re: prove limit by definition
Quote:

Originally Posted by transgalactic
prove that $lim_{x\rightarrow2}\frac{x}{x+1}\neq1$ by epsilon delta

defintion.
we need to prove that their is $\varepsilon>0$ so for every $\delta>0$
their is x $|x-2|<\delta$ and $|\frac{x}{x+1}-1|\geq\varepsilon$
so we are free to choose our epsilon and our x
i choose $\varepsilon=2$
x cannot be equal to -1 because of the denominator.
and we need to choose x for which $|\frac{x}{x+1}-1|\geq2$
$|\frac{x-x-1}{x+1}|\geq2$ -> $|\frac{1}{x+1}|\geq2$ -> $\frac{1}{2}\geq|x+1|$
-> $-\frac{1}{2}\leq x+1\leq\frac{1}{2}$ -> $-\frac{3}{2}\leq x\leq-\frac{1}{2}$
but x=-1 there so we have a problem.
where is the problem in my way???

the prof solved it by $\varepsilon=1/6$
$2>x>max\{2-\delta/2,1\}$
cant understand why he choose such x

Since the actual limit is 2/3, you will always be able to show that for x close enough to 2 that |x/(x+1)-1|>0.3, so you need to choose epsilon greater than zero but less than 0.3 (well actually any thing less than 1/3 will do). Then show that for all delta>0 there is always some x such that |x-1|<delta and |x/(x+1) - 1|>epsilon.

CB
• August 9th 2011, 02:24 AM
transgalactic
Re: prove limit by definition
what is the problem of my way.
??
• August 9th 2011, 06:08 AM
Duke
Re: prove limit by definition
Consider ${\delta}$ <_1. Then if x satisfy's 0< |x-2|< ${\delta}$, F(x)<3/4. Hence |F(x)-1|>1/4. Choosing ${\epsilon}$<_1/4 completes the proof.
• August 9th 2011, 07:42 AM
transgalactic
Re: prove limit by definition
i cant because its for every delta

and can you tell me where am i wrong
?
• August 9th 2011, 08:02 AM
Duke
Re: prove limit by definition
For any delta>1, x values in (1,3) will still have to satisfy |F(x)-1|< epsilon.
• August 9th 2011, 07:57 PM
SammyS
Re: prove limit by definition
It's not true that you are totally free to choose any ε that you like.

As CaptainBlack told you, the actual limit is 2/3.

|1 - 2/3| = 1/3. So, if you use an ε ≥ 1/3, it is possible to find a δ > 0 such that for all x for which 0 < | x - 2 | < δ it follows that | x/(x+1) - 1 | < ε .

That's why CB said you need to choose ε < 1/3 .