How to prove that $lim_{xrightarrow0}frac{1}{2x+1}=1$ using $delta-epsilon$ definition?
$begingroup$
How to prove that $limlimits_{xrightarrow 0}frac{1}{2x+1}=1$ using $delta-epsilon$ definition?
I have the following so far-
$|x-0|<delta$ and $left|frac{1}{2x+1}-1right|<epsilon$
which boils to $left|frac{-2x}{2x+1}right|<epsilon$
I'm trying to figure out a way to factor out an $x$ but don't know how
calculus limits proof-verification proof-writing epsilon-delta
$endgroup$
add a comment |
$begingroup$
How to prove that $limlimits_{xrightarrow 0}frac{1}{2x+1}=1$ using $delta-epsilon$ definition?
I have the following so far-
$|x-0|<delta$ and $left|frac{1}{2x+1}-1right|<epsilon$
which boils to $left|frac{-2x}{2x+1}right|<epsilon$
I'm trying to figure out a way to factor out an $x$ but don't know how
calculus limits proof-verification proof-writing epsilon-delta
$endgroup$
1
$begingroup$
@obscurans Thanks for you effort to edit posts into better shape. I'll just mention that you can uselim
to get nicer rendering and also proper spacing for the limit symbol. For example$lim_{ntoinfty} x_n$
$lim_{ntoinfty} x_n$ or$limlimits_{ntoinfty} x_n$
$limlimits_{ntoinfty} x_n$.
$endgroup$
– Martin Sleziak
Dec 19 '18 at 5:41
add a comment |
$begingroup$
How to prove that $limlimits_{xrightarrow 0}frac{1}{2x+1}=1$ using $delta-epsilon$ definition?
I have the following so far-
$|x-0|<delta$ and $left|frac{1}{2x+1}-1right|<epsilon$
which boils to $left|frac{-2x}{2x+1}right|<epsilon$
I'm trying to figure out a way to factor out an $x$ but don't know how
calculus limits proof-verification proof-writing epsilon-delta
$endgroup$
How to prove that $limlimits_{xrightarrow 0}frac{1}{2x+1}=1$ using $delta-epsilon$ definition?
I have the following so far-
$|x-0|<delta$ and $left|frac{1}{2x+1}-1right|<epsilon$
which boils to $left|frac{-2x}{2x+1}right|<epsilon$
I'm trying to figure out a way to factor out an $x$ but don't know how
calculus limits proof-verification proof-writing epsilon-delta
calculus limits proof-verification proof-writing epsilon-delta
edited Dec 19 '18 at 5:39
Martin Sleziak
44.7k10119272
44.7k10119272
asked Dec 19 '18 at 4:26
John RawlsJohn Rawls
1,253619
1,253619
1
$begingroup$
@obscurans Thanks for you effort to edit posts into better shape. I'll just mention that you can uselim
to get nicer rendering and also proper spacing for the limit symbol. For example$lim_{ntoinfty} x_n$
$lim_{ntoinfty} x_n$ or$limlimits_{ntoinfty} x_n$
$limlimits_{ntoinfty} x_n$.
$endgroup$
– Martin Sleziak
Dec 19 '18 at 5:41
add a comment |
1
$begingroup$
@obscurans Thanks for you effort to edit posts into better shape. I'll just mention that you can uselim
to get nicer rendering and also proper spacing for the limit symbol. For example$lim_{ntoinfty} x_n$
$lim_{ntoinfty} x_n$ or$limlimits_{ntoinfty} x_n$
$limlimits_{ntoinfty} x_n$.
$endgroup$
– Martin Sleziak
Dec 19 '18 at 5:41
1
1
$begingroup$
@obscurans Thanks for you effort to edit posts into better shape. I'll just mention that you can use
lim
to get nicer rendering and also proper spacing for the limit symbol. For example $lim_{ntoinfty} x_n$
$lim_{ntoinfty} x_n$ or $limlimits_{ntoinfty} x_n$
$limlimits_{ntoinfty} x_n$.$endgroup$
– Martin Sleziak
Dec 19 '18 at 5:41
$begingroup$
@obscurans Thanks for you effort to edit posts into better shape. I'll just mention that you can use
lim
to get nicer rendering and also proper spacing for the limit symbol. For example $lim_{ntoinfty} x_n$
$lim_{ntoinfty} x_n$ or $limlimits_{ntoinfty} x_n$
$limlimits_{ntoinfty} x_n$.$endgroup$
– Martin Sleziak
Dec 19 '18 at 5:41
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
So you want
$|dfrac{2x}{2x+1}|<epsilon
$.
There are two ways go here.
For the first,
note that you don't need
the best bounds,
just one that will work.
Soppose
$|x| < dfrac14$.
Then
$|2x| < dfrac12$
so
$|1+2x| > 1-2cdot dfrac14
= dfrac12$.
Therefore
$|dfrac{2x}{2x+1}|
lt |dfrac{2x}{frac12}|
=|4x|
$.
To make
$|4x| < epsilon$,
you just need
$|x| < dfrac{epsilon}{4}$.
Therefore,
if $|x| < min(dfrac14, dfrac{epsilon}{4})$,
we have
$|dfrac{2x}{2x+1}|<epsilon
$.
If you want the best bounds,
$|dfrac{2x}{2x+1}|<epsilon
$
is the same as
$dfrac1{epsilon}
lt |dfrac{2x+1}{2x}|
= |1+dfrac{1}{2x}|
$.
If $x > 0$,
$|1+dfrac{1}{2x}|
=1+dfrac{1}{2x}
$
so we want
$dfrac1{epsilon}
lt 1+dfrac{1}{2x}
$
or
$dfrac1{epsilon}-1
lt dfrac{1}{2x}
$
or,
assuming $epsilon < 1$,
$2x
lt dfrac1{dfrac1{epsilon}-1}
= dfrac{epsilon}{1-epsilon}
$
or
$x
ltdfrac{epsilon}{2(1-epsilon)}
$.
If $x < 0$,
to make
$dfrac1{epsilon}
lt |1+dfrac{1}{2x}|
$
be easy to work with,
we need
$1+dfrac{1}{2x} < 0$
or
$x > -dfrac12$.
If this holds,
then
$|1+dfrac{1}{2x}|
=-dfrac{1}{2x}-1
$,
so we want
$-dfrac{1}{2x}-1
gt dfrac1{epsilon}
$
or
$-dfrac{1}{2x}
gt dfrac1{epsilon}+1
$
or
$-2x
lt dfrac1{dfrac1{epsilon}+1}
$
or
$-x
lt dfrac{epsilon}{2(1+epsilon)}
$.
I always prefer to get
a simple bound
which is not the best.
$endgroup$
add a comment |
$begingroup$
$midfrac{-2x}{2x+1}mid=midfrac{-2}{2+frac1x}midltmidfrac2{frac 1x}midltepsilon$. So make $mid xmidltfrac{epsilon}2$. That is, $delta =frac {epsilon}2$.
$endgroup$
add a comment |
$begingroup$
For $quaddelta<dfrac{varepsilon}{2(1-varepsilon)}impliesquaddisplaystyleleft|dfrac{-2x}{2x+1}right|<dfrac{2delta}{2delta+1}=dfrac{1}{1+frac{1}{2delta}}<varepsilon$
$endgroup$
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
So you want
$|dfrac{2x}{2x+1}|<epsilon
$.
There are two ways go here.
For the first,
note that you don't need
the best bounds,
just one that will work.
Soppose
$|x| < dfrac14$.
Then
$|2x| < dfrac12$
so
$|1+2x| > 1-2cdot dfrac14
= dfrac12$.
Therefore
$|dfrac{2x}{2x+1}|
lt |dfrac{2x}{frac12}|
=|4x|
$.
To make
$|4x| < epsilon$,
you just need
$|x| < dfrac{epsilon}{4}$.
Therefore,
if $|x| < min(dfrac14, dfrac{epsilon}{4})$,
we have
$|dfrac{2x}{2x+1}|<epsilon
$.
If you want the best bounds,
$|dfrac{2x}{2x+1}|<epsilon
$
is the same as
$dfrac1{epsilon}
lt |dfrac{2x+1}{2x}|
= |1+dfrac{1}{2x}|
$.
If $x > 0$,
$|1+dfrac{1}{2x}|
=1+dfrac{1}{2x}
$
so we want
$dfrac1{epsilon}
lt 1+dfrac{1}{2x}
$
or
$dfrac1{epsilon}-1
lt dfrac{1}{2x}
$
or,
assuming $epsilon < 1$,
$2x
lt dfrac1{dfrac1{epsilon}-1}
= dfrac{epsilon}{1-epsilon}
$
or
$x
ltdfrac{epsilon}{2(1-epsilon)}
$.
If $x < 0$,
to make
$dfrac1{epsilon}
lt |1+dfrac{1}{2x}|
$
be easy to work with,
we need
$1+dfrac{1}{2x} < 0$
or
$x > -dfrac12$.
If this holds,
then
$|1+dfrac{1}{2x}|
=-dfrac{1}{2x}-1
$,
so we want
$-dfrac{1}{2x}-1
gt dfrac1{epsilon}
$
or
$-dfrac{1}{2x}
gt dfrac1{epsilon}+1
$
or
$-2x
lt dfrac1{dfrac1{epsilon}+1}
$
or
$-x
lt dfrac{epsilon}{2(1+epsilon)}
$.
I always prefer to get
a simple bound
which is not the best.
$endgroup$
add a comment |
$begingroup$
So you want
$|dfrac{2x}{2x+1}|<epsilon
$.
There are two ways go here.
For the first,
note that you don't need
the best bounds,
just one that will work.
Soppose
$|x| < dfrac14$.
Then
$|2x| < dfrac12$
so
$|1+2x| > 1-2cdot dfrac14
= dfrac12$.
Therefore
$|dfrac{2x}{2x+1}|
lt |dfrac{2x}{frac12}|
=|4x|
$.
To make
$|4x| < epsilon$,
you just need
$|x| < dfrac{epsilon}{4}$.
Therefore,
if $|x| < min(dfrac14, dfrac{epsilon}{4})$,
we have
$|dfrac{2x}{2x+1}|<epsilon
$.
If you want the best bounds,
$|dfrac{2x}{2x+1}|<epsilon
$
is the same as
$dfrac1{epsilon}
lt |dfrac{2x+1}{2x}|
= |1+dfrac{1}{2x}|
$.
If $x > 0$,
$|1+dfrac{1}{2x}|
=1+dfrac{1}{2x}
$
so we want
$dfrac1{epsilon}
lt 1+dfrac{1}{2x}
$
or
$dfrac1{epsilon}-1
lt dfrac{1}{2x}
$
or,
assuming $epsilon < 1$,
$2x
lt dfrac1{dfrac1{epsilon}-1}
= dfrac{epsilon}{1-epsilon}
$
or
$x
ltdfrac{epsilon}{2(1-epsilon)}
$.
If $x < 0$,
to make
$dfrac1{epsilon}
lt |1+dfrac{1}{2x}|
$
be easy to work with,
we need
$1+dfrac{1}{2x} < 0$
or
$x > -dfrac12$.
If this holds,
then
$|1+dfrac{1}{2x}|
=-dfrac{1}{2x}-1
$,
so we want
$-dfrac{1}{2x}-1
gt dfrac1{epsilon}
$
or
$-dfrac{1}{2x}
gt dfrac1{epsilon}+1
$
or
$-2x
lt dfrac1{dfrac1{epsilon}+1}
$
or
$-x
lt dfrac{epsilon}{2(1+epsilon)}
$.
I always prefer to get
a simple bound
which is not the best.
$endgroup$
add a comment |
$begingroup$
So you want
$|dfrac{2x}{2x+1}|<epsilon
$.
There are two ways go here.
For the first,
note that you don't need
the best bounds,
just one that will work.
Soppose
$|x| < dfrac14$.
Then
$|2x| < dfrac12$
so
$|1+2x| > 1-2cdot dfrac14
= dfrac12$.
Therefore
$|dfrac{2x}{2x+1}|
lt |dfrac{2x}{frac12}|
=|4x|
$.
To make
$|4x| < epsilon$,
you just need
$|x| < dfrac{epsilon}{4}$.
Therefore,
if $|x| < min(dfrac14, dfrac{epsilon}{4})$,
we have
$|dfrac{2x}{2x+1}|<epsilon
$.
If you want the best bounds,
$|dfrac{2x}{2x+1}|<epsilon
$
is the same as
$dfrac1{epsilon}
lt |dfrac{2x+1}{2x}|
= |1+dfrac{1}{2x}|
$.
If $x > 0$,
$|1+dfrac{1}{2x}|
=1+dfrac{1}{2x}
$
so we want
$dfrac1{epsilon}
lt 1+dfrac{1}{2x}
$
or
$dfrac1{epsilon}-1
lt dfrac{1}{2x}
$
or,
assuming $epsilon < 1$,
$2x
lt dfrac1{dfrac1{epsilon}-1}
= dfrac{epsilon}{1-epsilon}
$
or
$x
ltdfrac{epsilon}{2(1-epsilon)}
$.
If $x < 0$,
to make
$dfrac1{epsilon}
lt |1+dfrac{1}{2x}|
$
be easy to work with,
we need
$1+dfrac{1}{2x} < 0$
or
$x > -dfrac12$.
If this holds,
then
$|1+dfrac{1}{2x}|
=-dfrac{1}{2x}-1
$,
so we want
$-dfrac{1}{2x}-1
gt dfrac1{epsilon}
$
or
$-dfrac{1}{2x}
gt dfrac1{epsilon}+1
$
or
$-2x
lt dfrac1{dfrac1{epsilon}+1}
$
or
$-x
lt dfrac{epsilon}{2(1+epsilon)}
$.
I always prefer to get
a simple bound
which is not the best.
$endgroup$
So you want
$|dfrac{2x}{2x+1}|<epsilon
$.
There are two ways go here.
For the first,
note that you don't need
the best bounds,
just one that will work.
Soppose
$|x| < dfrac14$.
Then
$|2x| < dfrac12$
so
$|1+2x| > 1-2cdot dfrac14
= dfrac12$.
Therefore
$|dfrac{2x}{2x+1}|
lt |dfrac{2x}{frac12}|
=|4x|
$.
To make
$|4x| < epsilon$,
you just need
$|x| < dfrac{epsilon}{4}$.
Therefore,
if $|x| < min(dfrac14, dfrac{epsilon}{4})$,
we have
$|dfrac{2x}{2x+1}|<epsilon
$.
If you want the best bounds,
$|dfrac{2x}{2x+1}|<epsilon
$
is the same as
$dfrac1{epsilon}
lt |dfrac{2x+1}{2x}|
= |1+dfrac{1}{2x}|
$.
If $x > 0$,
$|1+dfrac{1}{2x}|
=1+dfrac{1}{2x}
$
so we want
$dfrac1{epsilon}
lt 1+dfrac{1}{2x}
$
or
$dfrac1{epsilon}-1
lt dfrac{1}{2x}
$
or,
assuming $epsilon < 1$,
$2x
lt dfrac1{dfrac1{epsilon}-1}
= dfrac{epsilon}{1-epsilon}
$
or
$x
ltdfrac{epsilon}{2(1-epsilon)}
$.
If $x < 0$,
to make
$dfrac1{epsilon}
lt |1+dfrac{1}{2x}|
$
be easy to work with,
we need
$1+dfrac{1}{2x} < 0$
or
$x > -dfrac12$.
If this holds,
then
$|1+dfrac{1}{2x}|
=-dfrac{1}{2x}-1
$,
so we want
$-dfrac{1}{2x}-1
gt dfrac1{epsilon}
$
or
$-dfrac{1}{2x}
gt dfrac1{epsilon}+1
$
or
$-2x
lt dfrac1{dfrac1{epsilon}+1}
$
or
$-x
lt dfrac{epsilon}{2(1+epsilon)}
$.
I always prefer to get
a simple bound
which is not the best.
answered Dec 19 '18 at 4:56
marty cohenmarty cohen
73.9k549128
73.9k549128
add a comment |
add a comment |
$begingroup$
$midfrac{-2x}{2x+1}mid=midfrac{-2}{2+frac1x}midltmidfrac2{frac 1x}midltepsilon$. So make $mid xmidltfrac{epsilon}2$. That is, $delta =frac {epsilon}2$.
$endgroup$
add a comment |
$begingroup$
$midfrac{-2x}{2x+1}mid=midfrac{-2}{2+frac1x}midltmidfrac2{frac 1x}midltepsilon$. So make $mid xmidltfrac{epsilon}2$. That is, $delta =frac {epsilon}2$.
$endgroup$
add a comment |
$begingroup$
$midfrac{-2x}{2x+1}mid=midfrac{-2}{2+frac1x}midltmidfrac2{frac 1x}midltepsilon$. So make $mid xmidltfrac{epsilon}2$. That is, $delta =frac {epsilon}2$.
$endgroup$
$midfrac{-2x}{2x+1}mid=midfrac{-2}{2+frac1x}midltmidfrac2{frac 1x}midltepsilon$. So make $mid xmidltfrac{epsilon}2$. That is, $delta =frac {epsilon}2$.
edited Dec 19 '18 at 4:54
answered Dec 19 '18 at 4:49
Chris CusterChris Custer
13.8k3827
13.8k3827
add a comment |
add a comment |
$begingroup$
For $quaddelta<dfrac{varepsilon}{2(1-varepsilon)}impliesquaddisplaystyleleft|dfrac{-2x}{2x+1}right|<dfrac{2delta}{2delta+1}=dfrac{1}{1+frac{1}{2delta}}<varepsilon$
$endgroup$
add a comment |
$begingroup$
For $quaddelta<dfrac{varepsilon}{2(1-varepsilon)}impliesquaddisplaystyleleft|dfrac{-2x}{2x+1}right|<dfrac{2delta}{2delta+1}=dfrac{1}{1+frac{1}{2delta}}<varepsilon$
$endgroup$
add a comment |
$begingroup$
For $quaddelta<dfrac{varepsilon}{2(1-varepsilon)}impliesquaddisplaystyleleft|dfrac{-2x}{2x+1}right|<dfrac{2delta}{2delta+1}=dfrac{1}{1+frac{1}{2delta}}<varepsilon$
$endgroup$
For $quaddelta<dfrac{varepsilon}{2(1-varepsilon)}impliesquaddisplaystyleleft|dfrac{-2x}{2x+1}right|<dfrac{2delta}{2delta+1}=dfrac{1}{1+frac{1}{2delta}}<varepsilon$
edited Dec 19 '18 at 5:10
answered Dec 19 '18 at 4:48
Yadati KiranYadati Kiran
1,7911619
1,7911619
add a comment |
add a comment |
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1
$begingroup$
@obscurans Thanks for you effort to edit posts into better shape. I'll just mention that you can use
lim
to get nicer rendering and also proper spacing for the limit symbol. For example$lim_{ntoinfty} x_n$
$lim_{ntoinfty} x_n$ or$limlimits_{ntoinfty} x_n$
$limlimits_{ntoinfty} x_n$.$endgroup$
– Martin Sleziak
Dec 19 '18 at 5:41