How to prove that $lim_{xrightarrow0}frac{1}{2x+1}=1$ using $delta-epsilon$ definition?












0












$begingroup$


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










share|cite|improve this question











$endgroup$








  • 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
















0












$begingroup$


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










share|cite|improve this question











$endgroup$








  • 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














0












0








0


3



$begingroup$


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










share|cite|improve this question











$endgroup$




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






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








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 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














  • 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








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










3 Answers
3






active

oldest

votes


















2












$begingroup$

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.






share|cite|improve this answer









$endgroup$





















    0












    $begingroup$

    $midfrac{-2x}{2x+1}mid=midfrac{-2}{2+frac1x}midltmidfrac2{frac 1x}midltepsilon$. So make $mid xmidltfrac{epsilon}2$. That is, $delta =frac {epsilon}2$.






    share|cite|improve this answer











    $endgroup$





















      0












      $begingroup$

      For $quaddelta<dfrac{varepsilon}{2(1-varepsilon)}impliesquaddisplaystyleleft|dfrac{-2x}{2x+1}right|<dfrac{2delta}{2delta+1}=dfrac{1}{1+frac{1}{2delta}}<varepsilon$






      share|cite|improve this answer











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        3 Answers
        3






        active

        oldest

        votes








        3 Answers
        3






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        2












        $begingroup$

        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.






        share|cite|improve this answer









        $endgroup$


















          2












          $begingroup$

          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.






          share|cite|improve this answer









          $endgroup$
















            2












            2








            2





            $begingroup$

            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.






            share|cite|improve this answer









            $endgroup$



            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.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Dec 19 '18 at 4:56









            marty cohenmarty cohen

            73.9k549128




            73.9k549128























                0












                $begingroup$

                $midfrac{-2x}{2x+1}mid=midfrac{-2}{2+frac1x}midltmidfrac2{frac 1x}midltepsilon$. So make $mid xmidltfrac{epsilon}2$. That is, $delta =frac {epsilon}2$.






                share|cite|improve this answer











                $endgroup$


















                  0












                  $begingroup$

                  $midfrac{-2x}{2x+1}mid=midfrac{-2}{2+frac1x}midltmidfrac2{frac 1x}midltepsilon$. So make $mid xmidltfrac{epsilon}2$. That is, $delta =frac {epsilon}2$.






                  share|cite|improve this answer











                  $endgroup$
















                    0












                    0








                    0





                    $begingroup$

                    $midfrac{-2x}{2x+1}mid=midfrac{-2}{2+frac1x}midltmidfrac2{frac 1x}midltepsilon$. So make $mid xmidltfrac{epsilon}2$. That is, $delta =frac {epsilon}2$.






                    share|cite|improve this answer











                    $endgroup$



                    $midfrac{-2x}{2x+1}mid=midfrac{-2}{2+frac1x}midltmidfrac2{frac 1x}midltepsilon$. So make $mid xmidltfrac{epsilon}2$. That is, $delta =frac {epsilon}2$.







                    share|cite|improve this answer














                    share|cite|improve this answer



                    share|cite|improve this answer








                    edited Dec 19 '18 at 4:54

























                    answered Dec 19 '18 at 4:49









                    Chris CusterChris Custer

                    13.8k3827




                    13.8k3827























                        0












                        $begingroup$

                        For $quaddelta<dfrac{varepsilon}{2(1-varepsilon)}impliesquaddisplaystyleleft|dfrac{-2x}{2x+1}right|<dfrac{2delta}{2delta+1}=dfrac{1}{1+frac{1}{2delta}}<varepsilon$






                        share|cite|improve this answer











                        $endgroup$


















                          0












                          $begingroup$

                          For $quaddelta<dfrac{varepsilon}{2(1-varepsilon)}impliesquaddisplaystyleleft|dfrac{-2x}{2x+1}right|<dfrac{2delta}{2delta+1}=dfrac{1}{1+frac{1}{2delta}}<varepsilon$






                          share|cite|improve this answer











                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            For $quaddelta<dfrac{varepsilon}{2(1-varepsilon)}impliesquaddisplaystyleleft|dfrac{-2x}{2x+1}right|<dfrac{2delta}{2delta+1}=dfrac{1}{1+frac{1}{2delta}}<varepsilon$






                            share|cite|improve this answer











                            $endgroup$



                            For $quaddelta<dfrac{varepsilon}{2(1-varepsilon)}impliesquaddisplaystyleleft|dfrac{-2x}{2x+1}right|<dfrac{2delta}{2delta+1}=dfrac{1}{1+frac{1}{2delta}}<varepsilon$







                            share|cite|improve this answer














                            share|cite|improve this answer



                            share|cite|improve this answer








                            edited Dec 19 '18 at 5:10

























                            answered Dec 19 '18 at 4:48









                            Yadati KiranYadati Kiran

                            1,7911619




                            1,7911619






























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