In $Delta ABC$ if $(sqrt{3}-1)a=2b$, $A=3B$, then find $C$












2












$begingroup$



In $Delta ABC$ if $(sqrt{3}-1)a=2b$, $A=3B$, then find $C$




My Attempt
$$
b=frac{sqrt{3}-1}{2}aquad& quad frac{A-B}{2}=Bquad&quadfrac{A+B}{2}=2B\frac{a-b}{a+b}=frac{tanfrac{A-B}{2}}{tanfrac{A+B}{2}}implies frac{3-sqrt{3}}{sqrt{3}+1}=frac{tan B}{tan 2B}=frac{1-tan^2B}{2}
$$






geometry trigonometry euclidean-geometry triangle







share|cite|improve this question











$endgroup$

















    2












    $begingroup$



    In $Delta ABC$ if $(sqrt{3}-1)a=2b$, $A=3B$, then find $C$




    My Attempt
    $$
    b=frac{sqrt{3}-1}{2}aquad& quad frac{A-B}{2}=Bquad&quadfrac{A+B}{2}=2B\frac{a-b}{a+b}=frac{tanfrac{A-B}{2}}{tanfrac{A+B}{2}}implies frac{3-sqrt{3}}{sqrt{3}+1}=frac{tan B}{tan 2B}=frac{1-tan^2B}{2}
    $$






    geometry trigonometry euclidean-geometry triangle







    share|cite|improve this question











    $endgroup$















      2












      2








      2


      1



      $begingroup$



      In $Delta ABC$ if $(sqrt{3}-1)a=2b$, $A=3B$, then find $C$




      My Attempt
      $$
      b=frac{sqrt{3}-1}{2}aquad& quad frac{A-B}{2}=Bquad&quadfrac{A+B}{2}=2B\frac{a-b}{a+b}=frac{tanfrac{A-B}{2}}{tanfrac{A+B}{2}}implies frac{3-sqrt{3}}{sqrt{3}+1}=frac{tan B}{tan 2B}=frac{1-tan^2B}{2}
      $$






      geometry trigonometry euclidean-geometry triangle







      share|cite|improve this question











      $endgroup$





      In $Delta ABC$ if $(sqrt{3}-1)a=2b$, $A=3B$, then find $C$




      My Attempt
      $$
      b=frac{sqrt{3}-1}{2}aquad& quad frac{A-B}{2}=Bquad&quadfrac{A+B}{2}=2B\frac{a-b}{a+b}=frac{tanfrac{A-B}{2}}{tanfrac{A+B}{2}}implies frac{3-sqrt{3}}{sqrt{3}+1}=frac{tan B}{tan 2B}=frac{1-tan^2B}{2}
      $$






      geometry trigonometry euclidean-geometry triangle




      geometry trigonometry euclidean-geometry triangle






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Jan 4 at 21:49









      Michael Rozenberg

      101k1591193




      101k1591193










      asked Jan 4 at 21:21









      ss1729ss1729

      1,9261723




      1,9261723






















          2 Answers
          2






          active

          oldest

          votes


















          2












          $begingroup$

          In the standard notation by law of sines we obtain:
          $$frac{sinbeta}{sin3beta}=frac{sqrt3-1}{2}$$ or
          $$frac{1}{3-4sin^2beta}=frac{sqrt3-1}{2}$$ or
          $$3-4sin^2beta=sqrt3+1$$ or
          $$8sin^2beta=(sqrt3-1)^2$$ or
          $$sinbeta=frac{sqrt3-1}{2sqrt2},$$ which gives $$beta=15^{circ}.$$
          Can you end it now?






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            I would've gone with $ 2 - 4sin^2 B = sqrt{3} implies 2cos(2B) = sqrt{3} implies 2B = 30^circ $, since $sin 15^circ$ isn't really well-known
            $endgroup$
            – Dylan
            Jan 5 at 11:26












          • $begingroup$
            @Dylan Or maybe $2beta=330^{circ}$, which is $beta=165^{circ},$ which is impossible. I think everyone, which solved during his life one ore maybe two problems, knows what is it $sin15^{circ}.$
            $endgroup$
            – Michael Rozenberg
            Jan 5 at 12:40










          • $begingroup$
            Well yes, there would be 2 solutions, one of which will be ruled out since $3B < 180^circ$. I've never had to solve for $sin 15^circ$ so I don't know what you're talking about.
            $endgroup$
            – Dylan
            Jan 5 at 13:37












          • $begingroup$
            @Dylan There are also $sin18^{circ}$, $cos36^{circ}$, $tan15^{circ}$, $tan22.5^{circ}$ and more. It's better to know it.
            $endgroup$
            – Michael Rozenberg
            Jan 5 at 13:40












          • $begingroup$
            I suppose you're right, but I don't think it should be considered common knowledge.
            $endgroup$
            – Dylan
            Jan 5 at 13:46



















          1












          $begingroup$

          $$2sin B=(sqrt3-1)sin3B=(...)(sin B)(3-4sin^2B)$$



          As $sin B>0,$ $$sin^2B=dfrac{2-sqrt3}4$$



          $$cos2B=1-dfrac{2-sqrt3}2=cos30^circ$$



          $0<2B<360^circ,2B=360^circ npm30^circ$ for some integer $n$



          $implies2B=30^circ$






          share|cite|improve this answer









          $endgroup$













            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%2f3062125%2fin-delta-abc-if-sqrt3-1a-2b-a-3b-then-find-c%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown

























            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            2












            $begingroup$

            In the standard notation by law of sines we obtain:
            $$frac{sinbeta}{sin3beta}=frac{sqrt3-1}{2}$$ or
            $$frac{1}{3-4sin^2beta}=frac{sqrt3-1}{2}$$ or
            $$3-4sin^2beta=sqrt3+1$$ or
            $$8sin^2beta=(sqrt3-1)^2$$ or
            $$sinbeta=frac{sqrt3-1}{2sqrt2},$$ which gives $$beta=15^{circ}.$$
            Can you end it now?






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              I would've gone with $ 2 - 4sin^2 B = sqrt{3} implies 2cos(2B) = sqrt{3} implies 2B = 30^circ $, since $sin 15^circ$ isn't really well-known
              $endgroup$
              – Dylan
              Jan 5 at 11:26












            • $begingroup$
              @Dylan Or maybe $2beta=330^{circ}$, which is $beta=165^{circ},$ which is impossible. I think everyone, which solved during his life one ore maybe two problems, knows what is it $sin15^{circ}.$
              $endgroup$
              – Michael Rozenberg
              Jan 5 at 12:40










            • $begingroup$
              Well yes, there would be 2 solutions, one of which will be ruled out since $3B < 180^circ$. I've never had to solve for $sin 15^circ$ so I don't know what you're talking about.
              $endgroup$
              – Dylan
              Jan 5 at 13:37












            • $begingroup$
              @Dylan There are also $sin18^{circ}$, $cos36^{circ}$, $tan15^{circ}$, $tan22.5^{circ}$ and more. It's better to know it.
              $endgroup$
              – Michael Rozenberg
              Jan 5 at 13:40












            • $begingroup$
              I suppose you're right, but I don't think it should be considered common knowledge.
              $endgroup$
              – Dylan
              Jan 5 at 13:46
















            2












            $begingroup$

            In the standard notation by law of sines we obtain:
            $$frac{sinbeta}{sin3beta}=frac{sqrt3-1}{2}$$ or
            $$frac{1}{3-4sin^2beta}=frac{sqrt3-1}{2}$$ or
            $$3-4sin^2beta=sqrt3+1$$ or
            $$8sin^2beta=(sqrt3-1)^2$$ or
            $$sinbeta=frac{sqrt3-1}{2sqrt2},$$ which gives $$beta=15^{circ}.$$
            Can you end it now?






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              I would've gone with $ 2 - 4sin^2 B = sqrt{3} implies 2cos(2B) = sqrt{3} implies 2B = 30^circ $, since $sin 15^circ$ isn't really well-known
              $endgroup$
              – Dylan
              Jan 5 at 11:26












            • $begingroup$
              @Dylan Or maybe $2beta=330^{circ}$, which is $beta=165^{circ},$ which is impossible. I think everyone, which solved during his life one ore maybe two problems, knows what is it $sin15^{circ}.$
              $endgroup$
              – Michael Rozenberg
              Jan 5 at 12:40










            • $begingroup$
              Well yes, there would be 2 solutions, one of which will be ruled out since $3B < 180^circ$. I've never had to solve for $sin 15^circ$ so I don't know what you're talking about.
              $endgroup$
              – Dylan
              Jan 5 at 13:37












            • $begingroup$
              @Dylan There are also $sin18^{circ}$, $cos36^{circ}$, $tan15^{circ}$, $tan22.5^{circ}$ and more. It's better to know it.
              $endgroup$
              – Michael Rozenberg
              Jan 5 at 13:40












            • $begingroup$
              I suppose you're right, but I don't think it should be considered common knowledge.
              $endgroup$
              – Dylan
              Jan 5 at 13:46














            2












            2








            2





            $begingroup$

            In the standard notation by law of sines we obtain:
            $$frac{sinbeta}{sin3beta}=frac{sqrt3-1}{2}$$ or
            $$frac{1}{3-4sin^2beta}=frac{sqrt3-1}{2}$$ or
            $$3-4sin^2beta=sqrt3+1$$ or
            $$8sin^2beta=(sqrt3-1)^2$$ or
            $$sinbeta=frac{sqrt3-1}{2sqrt2},$$ which gives $$beta=15^{circ}.$$
            Can you end it now?






            share|cite|improve this answer









            $endgroup$



            In the standard notation by law of sines we obtain:
            $$frac{sinbeta}{sin3beta}=frac{sqrt3-1}{2}$$ or
            $$frac{1}{3-4sin^2beta}=frac{sqrt3-1}{2}$$ or
            $$3-4sin^2beta=sqrt3+1$$ or
            $$8sin^2beta=(sqrt3-1)^2$$ or
            $$sinbeta=frac{sqrt3-1}{2sqrt2},$$ which gives $$beta=15^{circ}.$$
            Can you end it now?







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Jan 4 at 21:28









            Michael RozenbergMichael Rozenberg

            101k1591193




            101k1591193












            • $begingroup$
              I would've gone with $ 2 - 4sin^2 B = sqrt{3} implies 2cos(2B) = sqrt{3} implies 2B = 30^circ $, since $sin 15^circ$ isn't really well-known
              $endgroup$
              – Dylan
              Jan 5 at 11:26












            • $begingroup$
              @Dylan Or maybe $2beta=330^{circ}$, which is $beta=165^{circ},$ which is impossible. I think everyone, which solved during his life one ore maybe two problems, knows what is it $sin15^{circ}.$
              $endgroup$
              – Michael Rozenberg
              Jan 5 at 12:40










            • $begingroup$
              Well yes, there would be 2 solutions, one of which will be ruled out since $3B < 180^circ$. I've never had to solve for $sin 15^circ$ so I don't know what you're talking about.
              $endgroup$
              – Dylan
              Jan 5 at 13:37












            • $begingroup$
              @Dylan There are also $sin18^{circ}$, $cos36^{circ}$, $tan15^{circ}$, $tan22.5^{circ}$ and more. It's better to know it.
              $endgroup$
              – Michael Rozenberg
              Jan 5 at 13:40












            • $begingroup$
              I suppose you're right, but I don't think it should be considered common knowledge.
              $endgroup$
              – Dylan
              Jan 5 at 13:46


















            • $begingroup$
              I would've gone with $ 2 - 4sin^2 B = sqrt{3} implies 2cos(2B) = sqrt{3} implies 2B = 30^circ $, since $sin 15^circ$ isn't really well-known
              $endgroup$
              – Dylan
              Jan 5 at 11:26












            • $begingroup$
              @Dylan Or maybe $2beta=330^{circ}$, which is $beta=165^{circ},$ which is impossible. I think everyone, which solved during his life one ore maybe two problems, knows what is it $sin15^{circ}.$
              $endgroup$
              – Michael Rozenberg
              Jan 5 at 12:40










            • $begingroup$
              Well yes, there would be 2 solutions, one of which will be ruled out since $3B < 180^circ$. I've never had to solve for $sin 15^circ$ so I don't know what you're talking about.
              $endgroup$
              – Dylan
              Jan 5 at 13:37












            • $begingroup$
              @Dylan There are also $sin18^{circ}$, $cos36^{circ}$, $tan15^{circ}$, $tan22.5^{circ}$ and more. It's better to know it.
              $endgroup$
              – Michael Rozenberg
              Jan 5 at 13:40












            • $begingroup$
              I suppose you're right, but I don't think it should be considered common knowledge.
              $endgroup$
              – Dylan
              Jan 5 at 13:46
















            $begingroup$
            I would've gone with $ 2 - 4sin^2 B = sqrt{3} implies 2cos(2B) = sqrt{3} implies 2B = 30^circ $, since $sin 15^circ$ isn't really well-known
            $endgroup$
            – Dylan
            Jan 5 at 11:26






            $begingroup$
            I would've gone with $ 2 - 4sin^2 B = sqrt{3} implies 2cos(2B) = sqrt{3} implies 2B = 30^circ $, since $sin 15^circ$ isn't really well-known
            $endgroup$
            – Dylan
            Jan 5 at 11:26














            $begingroup$
            @Dylan Or maybe $2beta=330^{circ}$, which is $beta=165^{circ},$ which is impossible. I think everyone, which solved during his life one ore maybe two problems, knows what is it $sin15^{circ}.$
            $endgroup$
            – Michael Rozenberg
            Jan 5 at 12:40




            $begingroup$
            @Dylan Or maybe $2beta=330^{circ}$, which is $beta=165^{circ},$ which is impossible. I think everyone, which solved during his life one ore maybe two problems, knows what is it $sin15^{circ}.$
            $endgroup$
            – Michael Rozenberg
            Jan 5 at 12:40












            $begingroup$
            Well yes, there would be 2 solutions, one of which will be ruled out since $3B < 180^circ$. I've never had to solve for $sin 15^circ$ so I don't know what you're talking about.
            $endgroup$
            – Dylan
            Jan 5 at 13:37






            $begingroup$
            Well yes, there would be 2 solutions, one of which will be ruled out since $3B < 180^circ$. I've never had to solve for $sin 15^circ$ so I don't know what you're talking about.
            $endgroup$
            – Dylan
            Jan 5 at 13:37














            $begingroup$
            @Dylan There are also $sin18^{circ}$, $cos36^{circ}$, $tan15^{circ}$, $tan22.5^{circ}$ and more. It's better to know it.
            $endgroup$
            – Michael Rozenberg
            Jan 5 at 13:40






            $begingroup$
            @Dylan There are also $sin18^{circ}$, $cos36^{circ}$, $tan15^{circ}$, $tan22.5^{circ}$ and more. It's better to know it.
            $endgroup$
            – Michael Rozenberg
            Jan 5 at 13:40














            $begingroup$
            I suppose you're right, but I don't think it should be considered common knowledge.
            $endgroup$
            – Dylan
            Jan 5 at 13:46




            $begingroup$
            I suppose you're right, but I don't think it should be considered common knowledge.
            $endgroup$
            – Dylan
            Jan 5 at 13:46











            1












            $begingroup$

            $$2sin B=(sqrt3-1)sin3B=(...)(sin B)(3-4sin^2B)$$



            As $sin B>0,$ $$sin^2B=dfrac{2-sqrt3}4$$



            $$cos2B=1-dfrac{2-sqrt3}2=cos30^circ$$



            $0<2B<360^circ,2B=360^circ npm30^circ$ for some integer $n$



            $implies2B=30^circ$






            share|cite|improve this answer









            $endgroup$


















              1












              $begingroup$

              $$2sin B=(sqrt3-1)sin3B=(...)(sin B)(3-4sin^2B)$$



              As $sin B>0,$ $$sin^2B=dfrac{2-sqrt3}4$$



              $$cos2B=1-dfrac{2-sqrt3}2=cos30^circ$$



              $0<2B<360^circ,2B=360^circ npm30^circ$ for some integer $n$



              $implies2B=30^circ$






              share|cite|improve this answer









              $endgroup$
















                1












                1








                1





                $begingroup$

                $$2sin B=(sqrt3-1)sin3B=(...)(sin B)(3-4sin^2B)$$



                As $sin B>0,$ $$sin^2B=dfrac{2-sqrt3}4$$



                $$cos2B=1-dfrac{2-sqrt3}2=cos30^circ$$



                $0<2B<360^circ,2B=360^circ npm30^circ$ for some integer $n$



                $implies2B=30^circ$






                share|cite|improve this answer









                $endgroup$



                $$2sin B=(sqrt3-1)sin3B=(...)(sin B)(3-4sin^2B)$$



                As $sin B>0,$ $$sin^2B=dfrac{2-sqrt3}4$$



                $$cos2B=1-dfrac{2-sqrt3}2=cos30^circ$$



                $0<2B<360^circ,2B=360^circ npm30^circ$ for some integer $n$



                $implies2B=30^circ$







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Jan 5 at 2:45









                lab bhattacharjeelab bhattacharjee

                225k15157275




                225k15157275






























                    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%2f3062125%2fin-delta-abc-if-sqrt3-1a-2b-a-3b-then-find-c%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

                    Image
                    Human spaceflight From Wikipedia, the free encyclopedia Jump to navigation Jump to search "Space traveler" redirects here. For other uses, see Space traveler (disambiguation). @media all and (max-width:720px){.mw-parser-output .tmulti>.thumbinner{width:100%!important;max-width:none!important}.mw-parser-output .tmulti .tsingle{float:none!important;max-width:none!important;width:100%!important;text-align:center}} Apollo 11 crewmember Buzz Aldrin on the Moon, 1969 International Space Station crewmember Tracy Caldwell Dyson views the Earth, 2010 Space Shuttle Discovery heads into space with a crew aboard, STS-121 in 2006 Inside a space suit on the Canadarm, 1993 Human spaceflight (also referred to as crewed spaceflight or manned spaceflight ) is space travel with a crew or passengers aboard the spacecraft. Spacecraft carrying people may be operated directly, by human crew, or it may be e...
                    Read more

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

                    Image
                    0 1 What exactly does the error message beginning with E: mean? I assume it has to do with the file structure on Ubuntu-on-Windows, but what exactly? The following package was automatically installed and is no longer required: os-prober Use 'apt-get autoremove' to remove it. The following extra packages will be installed: libxslt1.1 The following NEW packages will be installed: libxslt1.1 xmlstarlet 0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded. Need to get 435 kB of archives. After this operation, 1023 kB of additional disk space will be used. Get:1 http://archive.ubuntu.com/ubuntu/ trusty/main libxslt1.1 amd64 1.1.28-2build1 [145 kB] Get:2 http://archive.ubuntu.com/ubuntu/ trusty/universe xmlstarlet amd64 1.5.0-1 [290 kB] ...
                    Read more

                    張江高科駅

                    Image
                    張江高科駅 ちょうこうこうか Zhangjiang Hi-Tech Park 所在地 中国上海市浦東新区 所属事業者 上海地鉄運営有限公司（上海軌道交通・駅詳細） 上海浦東現代有軌交通有限公司（駅詳細） テンプレートを表示 張江高科駅 各種表記 繁体字： 張江高科站 簡体字： 张江高科站 拼音 ： Zhàngjiāng Gāokē Zhàn 発音： ヂャンジャン ガオクー ヂャン 英文： Zhangjiang Hi-Tech Park Station テンプレートを表示 張江高科駅 （ちょうこうこうか-えき）は中華人民共和国上海市浦東新区に位置する、上海軌道交通2号線の駅である。 なお、張江高科駅前にある張江軌道電車の 張江地鉄駅駅 についてもここで述べる。 目次 1 利用可能な鉄道路線 2 沿革 3 駅構造 3.1 上海軌道交通 3.2 張江軌道電車 4 駅周辺 5 隣の駅 6 注釈 7 関連項目 利用可能な鉄道路線 上海軌道交通 ■ 2号線 上海浦東現代有軌交通有限公司 ■ 張江軌道電車 沿革 高架駅時代の姿 2000年12月27日 - 2号線の駅が高架駅として開業。 2010年 1月1日 - 張江軌道電車の開業に伴い、張江地鉄駅駅（停留所）を設置。 2月13日 - 駅地下化に伴い高架駅の運用停止。 2月24日 - 10日間の閉鎖を経て地下新駅の開業。 2017年2月9日 - リニューアル工事を経て再開業。乗降客の増加に対応し、ホームへのエレベーター、エスカレーターを増設、券売機エリアを拡張した [1] 。 駅構造 上海軌道交通 上海軌道交通 張江高科駅 5号口 ちょうこうこうか Zhangjiang Hi-Tech Park ◄ 竜陽路 (2.60km) (1.61km) 金科路 ► 所在地 中国上海市浦東新区松涛路 祖冲之路 （中国語版） 北緯31度12分11秒 東経121度34分50秒  /  北緯31.2...
                    Read more