Given $x^2=2$ prove for any rational number $frac{p}{q} < x$,there exists $frac{m}{n}$ such that...












0














Without using limits or the definition of irrational numbers, how do you solve this? I was thinking proof by contradiction, but I keep running into problems.










share|cite|improve this question





























    0














    Without using limits or the definition of irrational numbers, how do you solve this? I was thinking proof by contradiction, but I keep running into problems.










    share|cite|improve this question



























      0












      0








      0


      1





      Without using limits or the definition of irrational numbers, how do you solve this? I was thinking proof by contradiction, but I keep running into problems.










      share|cite|improve this question















      Without using limits or the definition of irrational numbers, how do you solve this? I was thinking proof by contradiction, but I keep running into problems.







      real-analysis proof-writing real-numbers rational-numbers






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Nov 21 '18 at 4:17









      Martund

      1,405212




      1,405212










      asked Nov 21 '18 at 3:49









      Masie

      113




      113






















          4 Answers
          4






          active

          oldest

          votes


















          0














          There are many possible answers, but I believe this one complies with the restrictions imposed (assuming $p,q>0$): $$frac{m}{n} = frac{4pq}{p^2+2q^2}.$$




          1. Proof of $frac{p}{q} < frac{m}{n}$. By hypothesis $p^2 < 2q^2$ then $$frac{m}{n} = frac{4pq}{p^2+2q^2} > frac{4pq}{2q^2+2q^2} = frac{4pq}{4q^2} = frac{p}{q}.$$


          2. Proof of $frac{m}{n} < sqrt{2}$, or equivalently, $0 < 2n^2 - m^2$:
            $$2n^2 - m^2 = 2(p^2+2q^2)^2 - 16 p^4q^4 = 2 (2q^2 - p^2)^2 > 0.$$



          Q.E.D.






          share|cite|improve this answer





















          • My approach was to use Newton's method to solve the equation $x^2 = 1/2$, enter $q/p$ in the algorithm and interpret its output as $n/m$. Since $q/p > 1/sqrt{2}$ and $x^2-1/2$ is convex, then $n/m$ had to be between $1/sqrt{2}$ and $q/p$.
            – mlerma54
            Nov 21 '18 at 6:12












          • Thanks! This helped a lot!
            – Masie
            Nov 22 '18 at 2:30










          • Just out of curiosity, how did you come up with that value for m/n ?
            – Masie
            Nov 22 '18 at 20:35










          • @Masie I used Newton's method for finding approximate roots of $x^2 - 1/2$ - Wikipedia contains a detailed description of the method. If you start with $q/p$ as a first "guess", then you can take $n/m$ as the next approximation generated by the algorithm. I didn't use $x^2-2$ and $p/q$ because the next approximation would overshoot the root.
            – mlerma54
            Nov 23 '18 at 3:47










          • Ah! I see. Got it. Thanks
            – Masie
            Nov 27 '18 at 20:37



















          4














          Let $a$ be a rational, close to, but below $sqrt2$. Then $b=2/a$ is a rational,
          close to, but above $sqrt2$. Consider $c=frac12(a+b)$. That is rational
          and should be even closer to $sqrt2$. But it turns out that $c>sqrt2$. Why
          not try then $d=2/c$? Can you prove $a<d<sqrt2$?






          share|cite|improve this answer





























            0














            We don't need the definition of irrational numbers, but we know that x is real. Now, by density of rational numbers, we can say that for any real number not equal to x ($frac{p}{q}$ in particular) their exist a rational between the number and x. Call it $frac{m}{n}$ and we are done.



            Hope it is helpful.






            share|cite|improve this answer





















            • I sincerely doubt that the density of rational numbers nor the existence of a real $sqrt 2$ is expected to be known for this exercise.
              – fleablood
              Nov 21 '18 at 4:25



















            0














            Let $frac{p}{q}=r$ and



            if $r>sqrt{2} - 1$, consider $(r+h)^2<2$ where $0<h<1$.



            Then $r^2+2rh+h^2<2.$ Let $r^2+2hr+h=2 <r^2+2rh+h^2$.



            Then $h=frac{2-r^2}{2r+1}<1$



            if $r<sqrt{2} - 1$, take $h=1$.






            share|cite|improve this answer





















              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%2f3007237%2fgiven-x2-2-prove-for-any-rational-number-fracpq-x-there-exists-fra%23new-answer', 'question_page');
              }
              );

              Post as a guest















              Required, but never shown

























              4 Answers
              4






              active

              oldest

              votes








              4 Answers
              4






              active

              oldest

              votes









              active

              oldest

              votes






              active

              oldest

              votes









              0














              There are many possible answers, but I believe this one complies with the restrictions imposed (assuming $p,q>0$): $$frac{m}{n} = frac{4pq}{p^2+2q^2}.$$




              1. Proof of $frac{p}{q} < frac{m}{n}$. By hypothesis $p^2 < 2q^2$ then $$frac{m}{n} = frac{4pq}{p^2+2q^2} > frac{4pq}{2q^2+2q^2} = frac{4pq}{4q^2} = frac{p}{q}.$$


              2. Proof of $frac{m}{n} < sqrt{2}$, or equivalently, $0 < 2n^2 - m^2$:
                $$2n^2 - m^2 = 2(p^2+2q^2)^2 - 16 p^4q^4 = 2 (2q^2 - p^2)^2 > 0.$$



              Q.E.D.






              share|cite|improve this answer





















              • My approach was to use Newton's method to solve the equation $x^2 = 1/2$, enter $q/p$ in the algorithm and interpret its output as $n/m$. Since $q/p > 1/sqrt{2}$ and $x^2-1/2$ is convex, then $n/m$ had to be between $1/sqrt{2}$ and $q/p$.
                – mlerma54
                Nov 21 '18 at 6:12












              • Thanks! This helped a lot!
                – Masie
                Nov 22 '18 at 2:30










              • Just out of curiosity, how did you come up with that value for m/n ?
                – Masie
                Nov 22 '18 at 20:35










              • @Masie I used Newton's method for finding approximate roots of $x^2 - 1/2$ - Wikipedia contains a detailed description of the method. If you start with $q/p$ as a first "guess", then you can take $n/m$ as the next approximation generated by the algorithm. I didn't use $x^2-2$ and $p/q$ because the next approximation would overshoot the root.
                – mlerma54
                Nov 23 '18 at 3:47










              • Ah! I see. Got it. Thanks
                – Masie
                Nov 27 '18 at 20:37
















              0














              There are many possible answers, but I believe this one complies with the restrictions imposed (assuming $p,q>0$): $$frac{m}{n} = frac{4pq}{p^2+2q^2}.$$




              1. Proof of $frac{p}{q} < frac{m}{n}$. By hypothesis $p^2 < 2q^2$ then $$frac{m}{n} = frac{4pq}{p^2+2q^2} > frac{4pq}{2q^2+2q^2} = frac{4pq}{4q^2} = frac{p}{q}.$$


              2. Proof of $frac{m}{n} < sqrt{2}$, or equivalently, $0 < 2n^2 - m^2$:
                $$2n^2 - m^2 = 2(p^2+2q^2)^2 - 16 p^4q^4 = 2 (2q^2 - p^2)^2 > 0.$$



              Q.E.D.






              share|cite|improve this answer





















              • My approach was to use Newton's method to solve the equation $x^2 = 1/2$, enter $q/p$ in the algorithm and interpret its output as $n/m$. Since $q/p > 1/sqrt{2}$ and $x^2-1/2$ is convex, then $n/m$ had to be between $1/sqrt{2}$ and $q/p$.
                – mlerma54
                Nov 21 '18 at 6:12












              • Thanks! This helped a lot!
                – Masie
                Nov 22 '18 at 2:30










              • Just out of curiosity, how did you come up with that value for m/n ?
                – Masie
                Nov 22 '18 at 20:35










              • @Masie I used Newton's method for finding approximate roots of $x^2 - 1/2$ - Wikipedia contains a detailed description of the method. If you start with $q/p$ as a first "guess", then you can take $n/m$ as the next approximation generated by the algorithm. I didn't use $x^2-2$ and $p/q$ because the next approximation would overshoot the root.
                – mlerma54
                Nov 23 '18 at 3:47










              • Ah! I see. Got it. Thanks
                – Masie
                Nov 27 '18 at 20:37














              0












              0








              0






              There are many possible answers, but I believe this one complies with the restrictions imposed (assuming $p,q>0$): $$frac{m}{n} = frac{4pq}{p^2+2q^2}.$$




              1. Proof of $frac{p}{q} < frac{m}{n}$. By hypothesis $p^2 < 2q^2$ then $$frac{m}{n} = frac{4pq}{p^2+2q^2} > frac{4pq}{2q^2+2q^2} = frac{4pq}{4q^2} = frac{p}{q}.$$


              2. Proof of $frac{m}{n} < sqrt{2}$, or equivalently, $0 < 2n^2 - m^2$:
                $$2n^2 - m^2 = 2(p^2+2q^2)^2 - 16 p^4q^4 = 2 (2q^2 - p^2)^2 > 0.$$



              Q.E.D.






              share|cite|improve this answer












              There are many possible answers, but I believe this one complies with the restrictions imposed (assuming $p,q>0$): $$frac{m}{n} = frac{4pq}{p^2+2q^2}.$$




              1. Proof of $frac{p}{q} < frac{m}{n}$. By hypothesis $p^2 < 2q^2$ then $$frac{m}{n} = frac{4pq}{p^2+2q^2} > frac{4pq}{2q^2+2q^2} = frac{4pq}{4q^2} = frac{p}{q}.$$


              2. Proof of $frac{m}{n} < sqrt{2}$, or equivalently, $0 < 2n^2 - m^2$:
                $$2n^2 - m^2 = 2(p^2+2q^2)^2 - 16 p^4q^4 = 2 (2q^2 - p^2)^2 > 0.$$



              Q.E.D.







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered Nov 21 '18 at 6:05









              mlerma54

              1,162138




              1,162138












              • My approach was to use Newton's method to solve the equation $x^2 = 1/2$, enter $q/p$ in the algorithm and interpret its output as $n/m$. Since $q/p > 1/sqrt{2}$ and $x^2-1/2$ is convex, then $n/m$ had to be between $1/sqrt{2}$ and $q/p$.
                – mlerma54
                Nov 21 '18 at 6:12












              • Thanks! This helped a lot!
                – Masie
                Nov 22 '18 at 2:30










              • Just out of curiosity, how did you come up with that value for m/n ?
                – Masie
                Nov 22 '18 at 20:35










              • @Masie I used Newton's method for finding approximate roots of $x^2 - 1/2$ - Wikipedia contains a detailed description of the method. If you start with $q/p$ as a first "guess", then you can take $n/m$ as the next approximation generated by the algorithm. I didn't use $x^2-2$ and $p/q$ because the next approximation would overshoot the root.
                – mlerma54
                Nov 23 '18 at 3:47










              • Ah! I see. Got it. Thanks
                – Masie
                Nov 27 '18 at 20:37


















              • My approach was to use Newton's method to solve the equation $x^2 = 1/2$, enter $q/p$ in the algorithm and interpret its output as $n/m$. Since $q/p > 1/sqrt{2}$ and $x^2-1/2$ is convex, then $n/m$ had to be between $1/sqrt{2}$ and $q/p$.
                – mlerma54
                Nov 21 '18 at 6:12












              • Thanks! This helped a lot!
                – Masie
                Nov 22 '18 at 2:30










              • Just out of curiosity, how did you come up with that value for m/n ?
                – Masie
                Nov 22 '18 at 20:35










              • @Masie I used Newton's method for finding approximate roots of $x^2 - 1/2$ - Wikipedia contains a detailed description of the method. If you start with $q/p$ as a first "guess", then you can take $n/m$ as the next approximation generated by the algorithm. I didn't use $x^2-2$ and $p/q$ because the next approximation would overshoot the root.
                – mlerma54
                Nov 23 '18 at 3:47










              • Ah! I see. Got it. Thanks
                – Masie
                Nov 27 '18 at 20:37
















              My approach was to use Newton's method to solve the equation $x^2 = 1/2$, enter $q/p$ in the algorithm and interpret its output as $n/m$. Since $q/p > 1/sqrt{2}$ and $x^2-1/2$ is convex, then $n/m$ had to be between $1/sqrt{2}$ and $q/p$.
              – mlerma54
              Nov 21 '18 at 6:12






              My approach was to use Newton's method to solve the equation $x^2 = 1/2$, enter $q/p$ in the algorithm and interpret its output as $n/m$. Since $q/p > 1/sqrt{2}$ and $x^2-1/2$ is convex, then $n/m$ had to be between $1/sqrt{2}$ and $q/p$.
              – mlerma54
              Nov 21 '18 at 6:12














              Thanks! This helped a lot!
              – Masie
              Nov 22 '18 at 2:30




              Thanks! This helped a lot!
              – Masie
              Nov 22 '18 at 2:30












              Just out of curiosity, how did you come up with that value for m/n ?
              – Masie
              Nov 22 '18 at 20:35




              Just out of curiosity, how did you come up with that value for m/n ?
              – Masie
              Nov 22 '18 at 20:35












              @Masie I used Newton's method for finding approximate roots of $x^2 - 1/2$ - Wikipedia contains a detailed description of the method. If you start with $q/p$ as a first "guess", then you can take $n/m$ as the next approximation generated by the algorithm. I didn't use $x^2-2$ and $p/q$ because the next approximation would overshoot the root.
              – mlerma54
              Nov 23 '18 at 3:47




              @Masie I used Newton's method for finding approximate roots of $x^2 - 1/2$ - Wikipedia contains a detailed description of the method. If you start with $q/p$ as a first "guess", then you can take $n/m$ as the next approximation generated by the algorithm. I didn't use $x^2-2$ and $p/q$ because the next approximation would overshoot the root.
              – mlerma54
              Nov 23 '18 at 3:47












              Ah! I see. Got it. Thanks
              – Masie
              Nov 27 '18 at 20:37




              Ah! I see. Got it. Thanks
              – Masie
              Nov 27 '18 at 20:37











              4














              Let $a$ be a rational, close to, but below $sqrt2$. Then $b=2/a$ is a rational,
              close to, but above $sqrt2$. Consider $c=frac12(a+b)$. That is rational
              and should be even closer to $sqrt2$. But it turns out that $c>sqrt2$. Why
              not try then $d=2/c$? Can you prove $a<d<sqrt2$?






              share|cite|improve this answer


























                4














                Let $a$ be a rational, close to, but below $sqrt2$. Then $b=2/a$ is a rational,
                close to, but above $sqrt2$. Consider $c=frac12(a+b)$. That is rational
                and should be even closer to $sqrt2$. But it turns out that $c>sqrt2$. Why
                not try then $d=2/c$? Can you prove $a<d<sqrt2$?






                share|cite|improve this answer
























                  4












                  4








                  4






                  Let $a$ be a rational, close to, but below $sqrt2$. Then $b=2/a$ is a rational,
                  close to, but above $sqrt2$. Consider $c=frac12(a+b)$. That is rational
                  and should be even closer to $sqrt2$. But it turns out that $c>sqrt2$. Why
                  not try then $d=2/c$? Can you prove $a<d<sqrt2$?






                  share|cite|improve this answer












                  Let $a$ be a rational, close to, but below $sqrt2$. Then $b=2/a$ is a rational,
                  close to, but above $sqrt2$. Consider $c=frac12(a+b)$. That is rational
                  and should be even closer to $sqrt2$. But it turns out that $c>sqrt2$. Why
                  not try then $d=2/c$? Can you prove $a<d<sqrt2$?







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Nov 21 '18 at 4:05









                  Lord Shark the Unknown

                  101k958132




                  101k958132























                      0














                      We don't need the definition of irrational numbers, but we know that x is real. Now, by density of rational numbers, we can say that for any real number not equal to x ($frac{p}{q}$ in particular) their exist a rational between the number and x. Call it $frac{m}{n}$ and we are done.



                      Hope it is helpful.






                      share|cite|improve this answer





















                      • I sincerely doubt that the density of rational numbers nor the existence of a real $sqrt 2$ is expected to be known for this exercise.
                        – fleablood
                        Nov 21 '18 at 4:25
















                      0














                      We don't need the definition of irrational numbers, but we know that x is real. Now, by density of rational numbers, we can say that for any real number not equal to x ($frac{p}{q}$ in particular) their exist a rational between the number and x. Call it $frac{m}{n}$ and we are done.



                      Hope it is helpful.






                      share|cite|improve this answer





















                      • I sincerely doubt that the density of rational numbers nor the existence of a real $sqrt 2$ is expected to be known for this exercise.
                        – fleablood
                        Nov 21 '18 at 4:25














                      0












                      0








                      0






                      We don't need the definition of irrational numbers, but we know that x is real. Now, by density of rational numbers, we can say that for any real number not equal to x ($frac{p}{q}$ in particular) their exist a rational between the number and x. Call it $frac{m}{n}$ and we are done.



                      Hope it is helpful.






                      share|cite|improve this answer












                      We don't need the definition of irrational numbers, but we know that x is real. Now, by density of rational numbers, we can say that for any real number not equal to x ($frac{p}{q}$ in particular) their exist a rational between the number and x. Call it $frac{m}{n}$ and we are done.



                      Hope it is helpful.







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered Nov 21 '18 at 4:05









                      Martund

                      1,405212




                      1,405212












                      • I sincerely doubt that the density of rational numbers nor the existence of a real $sqrt 2$ is expected to be known for this exercise.
                        – fleablood
                        Nov 21 '18 at 4:25


















                      • I sincerely doubt that the density of rational numbers nor the existence of a real $sqrt 2$ is expected to be known for this exercise.
                        – fleablood
                        Nov 21 '18 at 4:25
















                      I sincerely doubt that the density of rational numbers nor the existence of a real $sqrt 2$ is expected to be known for this exercise.
                      – fleablood
                      Nov 21 '18 at 4:25




                      I sincerely doubt that the density of rational numbers nor the existence of a real $sqrt 2$ is expected to be known for this exercise.
                      – fleablood
                      Nov 21 '18 at 4:25











                      0














                      Let $frac{p}{q}=r$ and



                      if $r>sqrt{2} - 1$, consider $(r+h)^2<2$ where $0<h<1$.



                      Then $r^2+2rh+h^2<2.$ Let $r^2+2hr+h=2 <r^2+2rh+h^2$.



                      Then $h=frac{2-r^2}{2r+1}<1$



                      if $r<sqrt{2} - 1$, take $h=1$.






                      share|cite|improve this answer


























                        0














                        Let $frac{p}{q}=r$ and



                        if $r>sqrt{2} - 1$, consider $(r+h)^2<2$ where $0<h<1$.



                        Then $r^2+2rh+h^2<2.$ Let $r^2+2hr+h=2 <r^2+2rh+h^2$.



                        Then $h=frac{2-r^2}{2r+1}<1$



                        if $r<sqrt{2} - 1$, take $h=1$.






                        share|cite|improve this answer
























                          0












                          0








                          0






                          Let $frac{p}{q}=r$ and



                          if $r>sqrt{2} - 1$, consider $(r+h)^2<2$ where $0<h<1$.



                          Then $r^2+2rh+h^2<2.$ Let $r^2+2hr+h=2 <r^2+2rh+h^2$.



                          Then $h=frac{2-r^2}{2r+1}<1$



                          if $r<sqrt{2} - 1$, take $h=1$.






                          share|cite|improve this answer












                          Let $frac{p}{q}=r$ and



                          if $r>sqrt{2} - 1$, consider $(r+h)^2<2$ where $0<h<1$.



                          Then $r^2+2rh+h^2<2.$ Let $r^2+2hr+h=2 <r^2+2rh+h^2$.



                          Then $h=frac{2-r^2}{2r+1}<1$



                          if $r<sqrt{2} - 1$, take $h=1$.







                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered Nov 21 '18 at 4:28









                          Offlaw

                          2649




                          2649






























                              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.





                              Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


                              Please pay close attention to the following guidance:


                              • 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.


                              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%2f3007237%2fgiven-x2-2-prove-for-any-rational-number-fracpq-x-there-exists-fra%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

                              android studio warns about leanback feature tag usage required on manifest while using Unity exported app?

                              SQL update select statement

                              'app-layout' is not a known element: how to share Component with different Modules