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












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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.










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    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



























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      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






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      edited Nov 21 '18 at 4:17









      Martund

      1,405212




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      asked Nov 21 '18 at 3:49









      Masie

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          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





















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              4 Answers
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              4 Answers
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              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






























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