Rationalizing denominator of $frac{7}{2+sqrt{3}}$. Cannot match textbook solution












2












$begingroup$


I am given this expression and asked to simplify by rationalizing the denominator:



$$frac{7}{2+sqrt{3}}$$



The solution is provided:



$14 - 7sqrt{3}$



I was able to get to this in the numerator but am left with a 4 in the denominator. Here are my steps:



$$frac{7}{2+sqrt{3}} * frac{2-sqrt{3}}{2-sqrt{3}}$$



=



$$frac{14 - 7sqrt{3}}{4}$$



Presumably I should not have a denominator here since the solution I'm given is just whats in the numerator. Presumably my numerator calculation is correct, where did I go wrong on the denominator?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    How did you compute $4$ in the denominator?
    $endgroup$
    – Martin R
    Jan 1 at 18:15










  • $begingroup$
    @MartinR my train of thought was 2 * 2 and then that $+sqrt{3}$ cancels out $-sqrt{3}$
    $endgroup$
    – Doug Fir
    Jan 1 at 18:18






  • 1




    $begingroup$
    Why the downvote???
    $endgroup$
    – Randall
    Jan 1 at 18:22
















2












$begingroup$


I am given this expression and asked to simplify by rationalizing the denominator:



$$frac{7}{2+sqrt{3}}$$



The solution is provided:



$14 - 7sqrt{3}$



I was able to get to this in the numerator but am left with a 4 in the denominator. Here are my steps:



$$frac{7}{2+sqrt{3}} * frac{2-sqrt{3}}{2-sqrt{3}}$$



=



$$frac{14 - 7sqrt{3}}{4}$$



Presumably I should not have a denominator here since the solution I'm given is just whats in the numerator. Presumably my numerator calculation is correct, where did I go wrong on the denominator?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    How did you compute $4$ in the denominator?
    $endgroup$
    – Martin R
    Jan 1 at 18:15










  • $begingroup$
    @MartinR my train of thought was 2 * 2 and then that $+sqrt{3}$ cancels out $-sqrt{3}$
    $endgroup$
    – Doug Fir
    Jan 1 at 18:18






  • 1




    $begingroup$
    Why the downvote???
    $endgroup$
    – Randall
    Jan 1 at 18:22














2












2








2





$begingroup$


I am given this expression and asked to simplify by rationalizing the denominator:



$$frac{7}{2+sqrt{3}}$$



The solution is provided:



$14 - 7sqrt{3}$



I was able to get to this in the numerator but am left with a 4 in the denominator. Here are my steps:



$$frac{7}{2+sqrt{3}} * frac{2-sqrt{3}}{2-sqrt{3}}$$



=



$$frac{14 - 7sqrt{3}}{4}$$



Presumably I should not have a denominator here since the solution I'm given is just whats in the numerator. Presumably my numerator calculation is correct, where did I go wrong on the denominator?










share|cite|improve this question











$endgroup$




I am given this expression and asked to simplify by rationalizing the denominator:



$$frac{7}{2+sqrt{3}}$$



The solution is provided:



$14 - 7sqrt{3}$



I was able to get to this in the numerator but am left with a 4 in the denominator. Here are my steps:



$$frac{7}{2+sqrt{3}} * frac{2-sqrt{3}}{2-sqrt{3}}$$



=



$$frac{14 - 7sqrt{3}}{4}$$



Presumably I should not have a denominator here since the solution I'm given is just whats in the numerator. Presumably my numerator calculation is correct, where did I go wrong on the denominator?







algebra-precalculus rationalising-denominator






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 1 at 18:27









José Carlos Santos

156k22125227




156k22125227










asked Jan 1 at 17:53









Doug FirDoug Fir

3227




3227








  • 1




    $begingroup$
    How did you compute $4$ in the denominator?
    $endgroup$
    – Martin R
    Jan 1 at 18:15










  • $begingroup$
    @MartinR my train of thought was 2 * 2 and then that $+sqrt{3}$ cancels out $-sqrt{3}$
    $endgroup$
    – Doug Fir
    Jan 1 at 18:18






  • 1




    $begingroup$
    Why the downvote???
    $endgroup$
    – Randall
    Jan 1 at 18:22














  • 1




    $begingroup$
    How did you compute $4$ in the denominator?
    $endgroup$
    – Martin R
    Jan 1 at 18:15










  • $begingroup$
    @MartinR my train of thought was 2 * 2 and then that $+sqrt{3}$ cancels out $-sqrt{3}$
    $endgroup$
    – Doug Fir
    Jan 1 at 18:18






  • 1




    $begingroup$
    Why the downvote???
    $endgroup$
    – Randall
    Jan 1 at 18:22








1




1




$begingroup$
How did you compute $4$ in the denominator?
$endgroup$
– Martin R
Jan 1 at 18:15




$begingroup$
How did you compute $4$ in the denominator?
$endgroup$
– Martin R
Jan 1 at 18:15












$begingroup$
@MartinR my train of thought was 2 * 2 and then that $+sqrt{3}$ cancels out $-sqrt{3}$
$endgroup$
– Doug Fir
Jan 1 at 18:18




$begingroup$
@MartinR my train of thought was 2 * 2 and then that $+sqrt{3}$ cancels out $-sqrt{3}$
$endgroup$
– Doug Fir
Jan 1 at 18:18




1




1




$begingroup$
Why the downvote???
$endgroup$
– Randall
Jan 1 at 18:22




$begingroup$
Why the downvote???
$endgroup$
– Randall
Jan 1 at 18:22










3 Answers
3






active

oldest

votes


















3












$begingroup$

Note that $left(2+sqrt3right)timesleft(2-sqrt3right)=4-3=1$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Hi, thanks for answering and sorry if my follow up sounds very low level but - why is it -3? I see how you get 4, from the product of both 2's in the denominator of both fractions being multiplied, but how do you get the -3 part?
    $endgroup$
    – Doug Fir
    Jan 1 at 17:57






  • 4




    $begingroup$
    @DougFir Difference of Two Squares: $a^2-b^2=(a+b)(a-b)$.
    $endgroup$
    – Lord Shark the Unknown
    Jan 1 at 17:58



















3












$begingroup$

note that:
$$(a+b)(c+d)=ac+ad+bc+bd$$
so in some cases it simplifies to:
$$(a+b)(a-b)=a^2-b^2$$
for you, $a=2$ and $b=sqrt{3}$ so $a^2-b^2=4-3=1$






share|cite|improve this answer









$endgroup$





















    3












    $begingroup$

    Trick to remember forever and use again and again.



    $(a + b)(a-b) = a(a-b) + b(a-b)=$



    $a^2 - ab + ab - b^2 = a^2 - b^2$.



    So



    1) Whenever you need to factor $a^2 - b^2$ it always factor to to $(a+b)(a-b)$



    and



    2) If you ever need to get rid of a radical sign in $a+sqrt b$ you can always multiple by $(a + sqrt b)(a - sqrt b) = a^2 - sqrt b^2 = a^2 -b$.



    So



    3) to deradicalize a $frac m{sqrt a + sqrt b} = frac {m(sqrt a - sqrt b)}{(sqrt a - sqrt b)} = frac {m(sqrt a - sqrt b)}{a - b}$.



    So:



    $frac {7}{2 + sqrt 3} = $



    $frac {7(2 - sqrt 3)}{(2+sqrt 3)(2 - sqrt 3)} =$



    $frac {7(2-sqrt 3)}{2^2 - sqrt 3^2} =$



    $frac {7(2-sqrt 3)}{4-3} =$



    $frac {7(2-sqrt 3)}{1} =$



    $7(2-sqrt 3)$.



    Learn to recognize $a^2 -b^2 = (a+b)(a-b)$ in all its forms, for all its uses and in all its directions.



    You will be using it for the REST OF YOUR LIFE!






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Thank you for the tip!
      $endgroup$
      – Doug Fir
      Jan 1 at 20:32











    Your Answer





    StackExchange.ifUsing("editor", function () {
    return StackExchange.using("mathjaxEditing", function () {
    StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
    StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
    });
    });
    }, "mathjax-editing");

    StackExchange.ready(function() {
    var channelOptions = {
    tags: "".split(" "),
    id: "69"
    };
    initTagRenderer("".split(" "), "".split(" "), channelOptions);

    StackExchange.using("externalEditor", function() {
    // Have to fire editor after snippets, if snippets enabled
    if (StackExchange.settings.snippets.snippetsEnabled) {
    StackExchange.using("snippets", function() {
    createEditor();
    });
    }
    else {
    createEditor();
    }
    });

    function createEditor() {
    StackExchange.prepareEditor({
    heartbeatType: 'answer',
    autoActivateHeartbeat: false,
    convertImagesToLinks: true,
    noModals: true,
    showLowRepImageUploadWarning: true,
    reputationToPostImages: 10,
    bindNavPrevention: true,
    postfix: "",
    imageUploader: {
    brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
    contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
    allowUrls: true
    },
    noCode: true, onDemand: true,
    discardSelector: ".discard-answer"
    ,immediatelyShowMarkdownHelp:true
    });


    }
    });














    draft saved

    draft discarded


















    StackExchange.ready(
    function () {
    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3058697%2frationalizing-denominator-of-frac72-sqrt3-cannot-match-textbook-solut%23new-answer', 'question_page');
    }
    );

    Post as a guest















    Required, but never shown

























    3 Answers
    3






    active

    oldest

    votes








    3 Answers
    3






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    3












    $begingroup$

    Note that $left(2+sqrt3right)timesleft(2-sqrt3right)=4-3=1$.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Hi, thanks for answering and sorry if my follow up sounds very low level but - why is it -3? I see how you get 4, from the product of both 2's in the denominator of both fractions being multiplied, but how do you get the -3 part?
      $endgroup$
      – Doug Fir
      Jan 1 at 17:57






    • 4




      $begingroup$
      @DougFir Difference of Two Squares: $a^2-b^2=(a+b)(a-b)$.
      $endgroup$
      – Lord Shark the Unknown
      Jan 1 at 17:58
















    3












    $begingroup$

    Note that $left(2+sqrt3right)timesleft(2-sqrt3right)=4-3=1$.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Hi, thanks for answering and sorry if my follow up sounds very low level but - why is it -3? I see how you get 4, from the product of both 2's in the denominator of both fractions being multiplied, but how do you get the -3 part?
      $endgroup$
      – Doug Fir
      Jan 1 at 17:57






    • 4




      $begingroup$
      @DougFir Difference of Two Squares: $a^2-b^2=(a+b)(a-b)$.
      $endgroup$
      – Lord Shark the Unknown
      Jan 1 at 17:58














    3












    3








    3





    $begingroup$

    Note that $left(2+sqrt3right)timesleft(2-sqrt3right)=4-3=1$.






    share|cite|improve this answer









    $endgroup$



    Note that $left(2+sqrt3right)timesleft(2-sqrt3right)=4-3=1$.







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Jan 1 at 17:54









    José Carlos SantosJosé Carlos Santos

    156k22125227




    156k22125227












    • $begingroup$
      Hi, thanks for answering and sorry if my follow up sounds very low level but - why is it -3? I see how you get 4, from the product of both 2's in the denominator of both fractions being multiplied, but how do you get the -3 part?
      $endgroup$
      – Doug Fir
      Jan 1 at 17:57






    • 4




      $begingroup$
      @DougFir Difference of Two Squares: $a^2-b^2=(a+b)(a-b)$.
      $endgroup$
      – Lord Shark the Unknown
      Jan 1 at 17:58


















    • $begingroup$
      Hi, thanks for answering and sorry if my follow up sounds very low level but - why is it -3? I see how you get 4, from the product of both 2's in the denominator of both fractions being multiplied, but how do you get the -3 part?
      $endgroup$
      – Doug Fir
      Jan 1 at 17:57






    • 4




      $begingroup$
      @DougFir Difference of Two Squares: $a^2-b^2=(a+b)(a-b)$.
      $endgroup$
      – Lord Shark the Unknown
      Jan 1 at 17:58
















    $begingroup$
    Hi, thanks for answering and sorry if my follow up sounds very low level but - why is it -3? I see how you get 4, from the product of both 2's in the denominator of both fractions being multiplied, but how do you get the -3 part?
    $endgroup$
    – Doug Fir
    Jan 1 at 17:57




    $begingroup$
    Hi, thanks for answering and sorry if my follow up sounds very low level but - why is it -3? I see how you get 4, from the product of both 2's in the denominator of both fractions being multiplied, but how do you get the -3 part?
    $endgroup$
    – Doug Fir
    Jan 1 at 17:57




    4




    4




    $begingroup$
    @DougFir Difference of Two Squares: $a^2-b^2=(a+b)(a-b)$.
    $endgroup$
    – Lord Shark the Unknown
    Jan 1 at 17:58




    $begingroup$
    @DougFir Difference of Two Squares: $a^2-b^2=(a+b)(a-b)$.
    $endgroup$
    – Lord Shark the Unknown
    Jan 1 at 17:58











    3












    $begingroup$

    note that:
    $$(a+b)(c+d)=ac+ad+bc+bd$$
    so in some cases it simplifies to:
    $$(a+b)(a-b)=a^2-b^2$$
    for you, $a=2$ and $b=sqrt{3}$ so $a^2-b^2=4-3=1$






    share|cite|improve this answer









    $endgroup$


















      3












      $begingroup$

      note that:
      $$(a+b)(c+d)=ac+ad+bc+bd$$
      so in some cases it simplifies to:
      $$(a+b)(a-b)=a^2-b^2$$
      for you, $a=2$ and $b=sqrt{3}$ so $a^2-b^2=4-3=1$






      share|cite|improve this answer









      $endgroup$
















        3












        3








        3





        $begingroup$

        note that:
        $$(a+b)(c+d)=ac+ad+bc+bd$$
        so in some cases it simplifies to:
        $$(a+b)(a-b)=a^2-b^2$$
        for you, $a=2$ and $b=sqrt{3}$ so $a^2-b^2=4-3=1$






        share|cite|improve this answer









        $endgroup$



        note that:
        $$(a+b)(c+d)=ac+ad+bc+bd$$
        so in some cases it simplifies to:
        $$(a+b)(a-b)=a^2-b^2$$
        for you, $a=2$ and $b=sqrt{3}$ so $a^2-b^2=4-3=1$







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Jan 1 at 18:31









        Henry LeeHenry Lee

        1,884219




        1,884219























            3












            $begingroup$

            Trick to remember forever and use again and again.



            $(a + b)(a-b) = a(a-b) + b(a-b)=$



            $a^2 - ab + ab - b^2 = a^2 - b^2$.



            So



            1) Whenever you need to factor $a^2 - b^2$ it always factor to to $(a+b)(a-b)$



            and



            2) If you ever need to get rid of a radical sign in $a+sqrt b$ you can always multiple by $(a + sqrt b)(a - sqrt b) = a^2 - sqrt b^2 = a^2 -b$.



            So



            3) to deradicalize a $frac m{sqrt a + sqrt b} = frac {m(sqrt a - sqrt b)}{(sqrt a - sqrt b)} = frac {m(sqrt a - sqrt b)}{a - b}$.



            So:



            $frac {7}{2 + sqrt 3} = $



            $frac {7(2 - sqrt 3)}{(2+sqrt 3)(2 - sqrt 3)} =$



            $frac {7(2-sqrt 3)}{2^2 - sqrt 3^2} =$



            $frac {7(2-sqrt 3)}{4-3} =$



            $frac {7(2-sqrt 3)}{1} =$



            $7(2-sqrt 3)$.



            Learn to recognize $a^2 -b^2 = (a+b)(a-b)$ in all its forms, for all its uses and in all its directions.



            You will be using it for the REST OF YOUR LIFE!






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              Thank you for the tip!
              $endgroup$
              – Doug Fir
              Jan 1 at 20:32
















            3












            $begingroup$

            Trick to remember forever and use again and again.



            $(a + b)(a-b) = a(a-b) + b(a-b)=$



            $a^2 - ab + ab - b^2 = a^2 - b^2$.



            So



            1) Whenever you need to factor $a^2 - b^2$ it always factor to to $(a+b)(a-b)$



            and



            2) If you ever need to get rid of a radical sign in $a+sqrt b$ you can always multiple by $(a + sqrt b)(a - sqrt b) = a^2 - sqrt b^2 = a^2 -b$.



            So



            3) to deradicalize a $frac m{sqrt a + sqrt b} = frac {m(sqrt a - sqrt b)}{(sqrt a - sqrt b)} = frac {m(sqrt a - sqrt b)}{a - b}$.



            So:



            $frac {7}{2 + sqrt 3} = $



            $frac {7(2 - sqrt 3)}{(2+sqrt 3)(2 - sqrt 3)} =$



            $frac {7(2-sqrt 3)}{2^2 - sqrt 3^2} =$



            $frac {7(2-sqrt 3)}{4-3} =$



            $frac {7(2-sqrt 3)}{1} =$



            $7(2-sqrt 3)$.



            Learn to recognize $a^2 -b^2 = (a+b)(a-b)$ in all its forms, for all its uses and in all its directions.



            You will be using it for the REST OF YOUR LIFE!






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              Thank you for the tip!
              $endgroup$
              – Doug Fir
              Jan 1 at 20:32














            3












            3








            3





            $begingroup$

            Trick to remember forever and use again and again.



            $(a + b)(a-b) = a(a-b) + b(a-b)=$



            $a^2 - ab + ab - b^2 = a^2 - b^2$.



            So



            1) Whenever you need to factor $a^2 - b^2$ it always factor to to $(a+b)(a-b)$



            and



            2) If you ever need to get rid of a radical sign in $a+sqrt b$ you can always multiple by $(a + sqrt b)(a - sqrt b) = a^2 - sqrt b^2 = a^2 -b$.



            So



            3) to deradicalize a $frac m{sqrt a + sqrt b} = frac {m(sqrt a - sqrt b)}{(sqrt a - sqrt b)} = frac {m(sqrt a - sqrt b)}{a - b}$.



            So:



            $frac {7}{2 + sqrt 3} = $



            $frac {7(2 - sqrt 3)}{(2+sqrt 3)(2 - sqrt 3)} =$



            $frac {7(2-sqrt 3)}{2^2 - sqrt 3^2} =$



            $frac {7(2-sqrt 3)}{4-3} =$



            $frac {7(2-sqrt 3)}{1} =$



            $7(2-sqrt 3)$.



            Learn to recognize $a^2 -b^2 = (a+b)(a-b)$ in all its forms, for all its uses and in all its directions.



            You will be using it for the REST OF YOUR LIFE!






            share|cite|improve this answer









            $endgroup$



            Trick to remember forever and use again and again.



            $(a + b)(a-b) = a(a-b) + b(a-b)=$



            $a^2 - ab + ab - b^2 = a^2 - b^2$.



            So



            1) Whenever you need to factor $a^2 - b^2$ it always factor to to $(a+b)(a-b)$



            and



            2) If you ever need to get rid of a radical sign in $a+sqrt b$ you can always multiple by $(a + sqrt b)(a - sqrt b) = a^2 - sqrt b^2 = a^2 -b$.



            So



            3) to deradicalize a $frac m{sqrt a + sqrt b} = frac {m(sqrt a - sqrt b)}{(sqrt a - sqrt b)} = frac {m(sqrt a - sqrt b)}{a - b}$.



            So:



            $frac {7}{2 + sqrt 3} = $



            $frac {7(2 - sqrt 3)}{(2+sqrt 3)(2 - sqrt 3)} =$



            $frac {7(2-sqrt 3)}{2^2 - sqrt 3^2} =$



            $frac {7(2-sqrt 3)}{4-3} =$



            $frac {7(2-sqrt 3)}{1} =$



            $7(2-sqrt 3)$.



            Learn to recognize $a^2 -b^2 = (a+b)(a-b)$ in all its forms, for all its uses and in all its directions.



            You will be using it for the REST OF YOUR LIFE!







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Jan 1 at 18:45









            fleabloodfleablood

            69.2k22685




            69.2k22685












            • $begingroup$
              Thank you for the tip!
              $endgroup$
              – Doug Fir
              Jan 1 at 20:32


















            • $begingroup$
              Thank you for the tip!
              $endgroup$
              – Doug Fir
              Jan 1 at 20:32
















            $begingroup$
            Thank you for the tip!
            $endgroup$
            – Doug Fir
            Jan 1 at 20:32




            $begingroup$
            Thank you for the tip!
            $endgroup$
            – Doug Fir
            Jan 1 at 20:32


















            draft saved

            draft discarded




















































            Thanks for contributing an answer to Mathematics Stack Exchange!


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            Use MathJax to format equations. MathJax reference.


            To learn more, see our tips on writing great answers.




            draft saved


            draft discarded














            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3058697%2frationalizing-denominator-of-frac72-sqrt3-cannot-match-textbook-solut%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown





















































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown

































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown







            Popular posts from this blog

            Human spaceflight

            Can not write log (Is /dev/pts mounted?) - openpty in Ubuntu-on-Windows?

            張江高科駅