Verification for a Proof By Contradiction concerning a 5-root being irrational The Next CEO of...

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Verification for a Proof By Contradiction concerning a 5-root being irrational



The Next CEO of Stack OverflowProof that the set of irrational numbers is dense in realsThe square root of a prime number is irrationalUnderstanding the proof of “$sqrt{2}$ is irrational” by contradiction.Why do we consider that $p$ & $q$ are co-primes when proving square root of a prime number is irrational?Proof by contradictionPlease critique my proof that $sqrt{12}$ is irrationalProof that, for every positive integer $n$, $log_7(n)$ is either integer or irrationalIs this a valid proof for $sqrt5$ being irrational?Is this proof of $sqrt{3}$ being an irrational number correct?Proof by contradiction that an expression is irrational












1












$begingroup$


I was asked to prove that for a non negative $m$ number, that is not a 5-th power of an integer, its 5-root, $sqrt[5]m$ is not a rational number.
Thanks for any feedback.



Proof By Contradiction:



Suppose Not,



Assume that for a non-negative $m$ number, that is not a 5-th power of an integer, its 5-root $sqrt[5]m$ is a rational number.



Being a rational number we can say:$$sqrt[5]m=frac{p}{q}quad text{for} ;p,qinBbb{Z};text{and}:qneq0$$
Then by algebra,$$m=left(frac{p}{q}right)^{5}Rightarrow m=frac{p^{5}}{q^{5}}$$
Consider when $frac{p}{q}=n$, $n inBbb Z$,



This is a contradiction with the fact that $m$ is not a 5-th power of an integer.



Hence, $sqrt[5]{m}$ is not a rational number.










share|cite|improve this question









$endgroup$








  • 1




    $begingroup$
    Your attempt stops making sense to me after $m=p^5/q^5$.
    $endgroup$
    – learner
    Mar 16 at 5:27






  • 1




    $begingroup$
    How can you be sure that $frac{p}{q}$ will be an integer $n$?
    $endgroup$
    – Sujit Bhattacharyya
    Mar 16 at 5:30






  • 1




    $begingroup$
    It's not quite that easy. For all you know, $mq^5$ could be the fifth power of an integer even though $m$ is not. You need to use the prime factorization of $m$, just as is done to prove $sqrt{2}$ is irrational.
    $endgroup$
    – Robert Shore
    Mar 16 at 5:31
















1












$begingroup$


I was asked to prove that for a non negative $m$ number, that is not a 5-th power of an integer, its 5-root, $sqrt[5]m$ is not a rational number.
Thanks for any feedback.



Proof By Contradiction:



Suppose Not,



Assume that for a non-negative $m$ number, that is not a 5-th power of an integer, its 5-root $sqrt[5]m$ is a rational number.



Being a rational number we can say:$$sqrt[5]m=frac{p}{q}quad text{for} ;p,qinBbb{Z};text{and}:qneq0$$
Then by algebra,$$m=left(frac{p}{q}right)^{5}Rightarrow m=frac{p^{5}}{q^{5}}$$
Consider when $frac{p}{q}=n$, $n inBbb Z$,



This is a contradiction with the fact that $m$ is not a 5-th power of an integer.



Hence, $sqrt[5]{m}$ is not a rational number.










share|cite|improve this question









$endgroup$








  • 1




    $begingroup$
    Your attempt stops making sense to me after $m=p^5/q^5$.
    $endgroup$
    – learner
    Mar 16 at 5:27






  • 1




    $begingroup$
    How can you be sure that $frac{p}{q}$ will be an integer $n$?
    $endgroup$
    – Sujit Bhattacharyya
    Mar 16 at 5:30






  • 1




    $begingroup$
    It's not quite that easy. For all you know, $mq^5$ could be the fifth power of an integer even though $m$ is not. You need to use the prime factorization of $m$, just as is done to prove $sqrt{2}$ is irrational.
    $endgroup$
    – Robert Shore
    Mar 16 at 5:31














1












1








1





$begingroup$


I was asked to prove that for a non negative $m$ number, that is not a 5-th power of an integer, its 5-root, $sqrt[5]m$ is not a rational number.
Thanks for any feedback.



Proof By Contradiction:



Suppose Not,



Assume that for a non-negative $m$ number, that is not a 5-th power of an integer, its 5-root $sqrt[5]m$ is a rational number.



Being a rational number we can say:$$sqrt[5]m=frac{p}{q}quad text{for} ;p,qinBbb{Z};text{and}:qneq0$$
Then by algebra,$$m=left(frac{p}{q}right)^{5}Rightarrow m=frac{p^{5}}{q^{5}}$$
Consider when $frac{p}{q}=n$, $n inBbb Z$,



This is a contradiction with the fact that $m$ is not a 5-th power of an integer.



Hence, $sqrt[5]{m}$ is not a rational number.










share|cite|improve this question









$endgroup$




I was asked to prove that for a non negative $m$ number, that is not a 5-th power of an integer, its 5-root, $sqrt[5]m$ is not a rational number.
Thanks for any feedback.



Proof By Contradiction:



Suppose Not,



Assume that for a non-negative $m$ number, that is not a 5-th power of an integer, its 5-root $sqrt[5]m$ is a rational number.



Being a rational number we can say:$$sqrt[5]m=frac{p}{q}quad text{for} ;p,qinBbb{Z};text{and}:qneq0$$
Then by algebra,$$m=left(frac{p}{q}right)^{5}Rightarrow m=frac{p^{5}}{q^{5}}$$
Consider when $frac{p}{q}=n$, $n inBbb Z$,



This is a contradiction with the fact that $m$ is not a 5-th power of an integer.



Hence, $sqrt[5]{m}$ is not a rational number.







real-analysis proof-verification






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Mar 16 at 5:18









MACMAC

283




283








  • 1




    $begingroup$
    Your attempt stops making sense to me after $m=p^5/q^5$.
    $endgroup$
    – learner
    Mar 16 at 5:27






  • 1




    $begingroup$
    How can you be sure that $frac{p}{q}$ will be an integer $n$?
    $endgroup$
    – Sujit Bhattacharyya
    Mar 16 at 5:30






  • 1




    $begingroup$
    It's not quite that easy. For all you know, $mq^5$ could be the fifth power of an integer even though $m$ is not. You need to use the prime factorization of $m$, just as is done to prove $sqrt{2}$ is irrational.
    $endgroup$
    – Robert Shore
    Mar 16 at 5:31














  • 1




    $begingroup$
    Your attempt stops making sense to me after $m=p^5/q^5$.
    $endgroup$
    – learner
    Mar 16 at 5:27






  • 1




    $begingroup$
    How can you be sure that $frac{p}{q}$ will be an integer $n$?
    $endgroup$
    – Sujit Bhattacharyya
    Mar 16 at 5:30






  • 1




    $begingroup$
    It's not quite that easy. For all you know, $mq^5$ could be the fifth power of an integer even though $m$ is not. You need to use the prime factorization of $m$, just as is done to prove $sqrt{2}$ is irrational.
    $endgroup$
    – Robert Shore
    Mar 16 at 5:31








1




1




$begingroup$
Your attempt stops making sense to me after $m=p^5/q^5$.
$endgroup$
– learner
Mar 16 at 5:27




$begingroup$
Your attempt stops making sense to me after $m=p^5/q^5$.
$endgroup$
– learner
Mar 16 at 5:27




1




1




$begingroup$
How can you be sure that $frac{p}{q}$ will be an integer $n$?
$endgroup$
– Sujit Bhattacharyya
Mar 16 at 5:30




$begingroup$
How can you be sure that $frac{p}{q}$ will be an integer $n$?
$endgroup$
– Sujit Bhattacharyya
Mar 16 at 5:30




1




1




$begingroup$
It's not quite that easy. For all you know, $mq^5$ could be the fifth power of an integer even though $m$ is not. You need to use the prime factorization of $m$, just as is done to prove $sqrt{2}$ is irrational.
$endgroup$
– Robert Shore
Mar 16 at 5:31




$begingroup$
It's not quite that easy. For all you know, $mq^5$ could be the fifth power of an integer even though $m$ is not. You need to use the prime factorization of $m$, just as is done to prove $sqrt{2}$ is irrational.
$endgroup$
– Robert Shore
Mar 16 at 5:31










2 Answers
2






active

oldest

votes


















1












$begingroup$

Well, you are on the right track, but you cannot consider when $p/q = n in mathbb Z$.



HINT



I guess you implicitly meant that $m$ is a non-negative integer. Then, we have:



$q^5 cdot m = p^5$, where $m, q, p$ are non-negative integers, and $p$, $q$ are coprime with each other, or $p=1$ or $q=1$.



Can you continue?






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Are you saying that from the fact m exists in Z+ and m>0, p,q >0,and that we can define p,q as coprime with each other. From this we have two cases: where p=1, or q=1?
    $endgroup$
    – MAC
    Mar 20 at 1:23








  • 1




    $begingroup$
    @MAC No, I mean since p/q is a rational number, then q,p are either coprime; or q = 1; or p = 1. For each scenario, you could check for your equation, and you’ll find that only when q=1 will make you equation solvable, but in that case, m is 5-th power of an integer
    $endgroup$
    – Yujie Zha
    Mar 20 at 1:26





















2












$begingroup$

You may assume (wlog) that $qgt 0$, so since $p^5/q^5=minBbb Z$, this implies $q^5mid p^5$. You can furthermore assume (wlog) that $gcd(p,q)=1$. This would imply that $qmid p$, so $1=gcd(p,q)=q$ but then $m=p^5$ leading to a contradiction that $m$ is a 5th power of an integer, namely $p$





In fact, this argument can be generalized to $sqrt[n]m$ where $m$ is not a perfect $n^text{th}$ power






share|cite|improve this answer









$endgroup$














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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    1












    $begingroup$

    Well, you are on the right track, but you cannot consider when $p/q = n in mathbb Z$.



    HINT



    I guess you implicitly meant that $m$ is a non-negative integer. Then, we have:



    $q^5 cdot m = p^5$, where $m, q, p$ are non-negative integers, and $p$, $q$ are coprime with each other, or $p=1$ or $q=1$.



    Can you continue?






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Are you saying that from the fact m exists in Z+ and m>0, p,q >0,and that we can define p,q as coprime with each other. From this we have two cases: where p=1, or q=1?
      $endgroup$
      – MAC
      Mar 20 at 1:23








    • 1




      $begingroup$
      @MAC No, I mean since p/q is a rational number, then q,p are either coprime; or q = 1; or p = 1. For each scenario, you could check for your equation, and you’ll find that only when q=1 will make you equation solvable, but in that case, m is 5-th power of an integer
      $endgroup$
      – Yujie Zha
      Mar 20 at 1:26


















    1












    $begingroup$

    Well, you are on the right track, but you cannot consider when $p/q = n in mathbb Z$.



    HINT



    I guess you implicitly meant that $m$ is a non-negative integer. Then, we have:



    $q^5 cdot m = p^5$, where $m, q, p$ are non-negative integers, and $p$, $q$ are coprime with each other, or $p=1$ or $q=1$.



    Can you continue?






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Are you saying that from the fact m exists in Z+ and m>0, p,q >0,and that we can define p,q as coprime with each other. From this we have two cases: where p=1, or q=1?
      $endgroup$
      – MAC
      Mar 20 at 1:23








    • 1




      $begingroup$
      @MAC No, I mean since p/q is a rational number, then q,p are either coprime; or q = 1; or p = 1. For each scenario, you could check for your equation, and you’ll find that only when q=1 will make you equation solvable, but in that case, m is 5-th power of an integer
      $endgroup$
      – Yujie Zha
      Mar 20 at 1:26
















    1












    1








    1





    $begingroup$

    Well, you are on the right track, but you cannot consider when $p/q = n in mathbb Z$.



    HINT



    I guess you implicitly meant that $m$ is a non-negative integer. Then, we have:



    $q^5 cdot m = p^5$, where $m, q, p$ are non-negative integers, and $p$, $q$ are coprime with each other, or $p=1$ or $q=1$.



    Can you continue?






    share|cite|improve this answer









    $endgroup$



    Well, you are on the right track, but you cannot consider when $p/q = n in mathbb Z$.



    HINT



    I guess you implicitly meant that $m$ is a non-negative integer. Then, we have:



    $q^5 cdot m = p^5$, where $m, q, p$ are non-negative integers, and $p$, $q$ are coprime with each other, or $p=1$ or $q=1$.



    Can you continue?







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Mar 16 at 5:36









    Yujie ZhaYujie Zha

    6,98611729




    6,98611729












    • $begingroup$
      Are you saying that from the fact m exists in Z+ and m>0, p,q >0,and that we can define p,q as coprime with each other. From this we have two cases: where p=1, or q=1?
      $endgroup$
      – MAC
      Mar 20 at 1:23








    • 1




      $begingroup$
      @MAC No, I mean since p/q is a rational number, then q,p are either coprime; or q = 1; or p = 1. For each scenario, you could check for your equation, and you’ll find that only when q=1 will make you equation solvable, but in that case, m is 5-th power of an integer
      $endgroup$
      – Yujie Zha
      Mar 20 at 1:26




















    • $begingroup$
      Are you saying that from the fact m exists in Z+ and m>0, p,q >0,and that we can define p,q as coprime with each other. From this we have two cases: where p=1, or q=1?
      $endgroup$
      – MAC
      Mar 20 at 1:23








    • 1




      $begingroup$
      @MAC No, I mean since p/q is a rational number, then q,p are either coprime; or q = 1; or p = 1. For each scenario, you could check for your equation, and you’ll find that only when q=1 will make you equation solvable, but in that case, m is 5-th power of an integer
      $endgroup$
      – Yujie Zha
      Mar 20 at 1:26


















    $begingroup$
    Are you saying that from the fact m exists in Z+ and m>0, p,q >0,and that we can define p,q as coprime with each other. From this we have two cases: where p=1, or q=1?
    $endgroup$
    – MAC
    Mar 20 at 1:23






    $begingroup$
    Are you saying that from the fact m exists in Z+ and m>0, p,q >0,and that we can define p,q as coprime with each other. From this we have two cases: where p=1, or q=1?
    $endgroup$
    – MAC
    Mar 20 at 1:23






    1




    1




    $begingroup$
    @MAC No, I mean since p/q is a rational number, then q,p are either coprime; or q = 1; or p = 1. For each scenario, you could check for your equation, and you’ll find that only when q=1 will make you equation solvable, but in that case, m is 5-th power of an integer
    $endgroup$
    – Yujie Zha
    Mar 20 at 1:26






    $begingroup$
    @MAC No, I mean since p/q is a rational number, then q,p are either coprime; or q = 1; or p = 1. For each scenario, you could check for your equation, and you’ll find that only when q=1 will make you equation solvable, but in that case, m is 5-th power of an integer
    $endgroup$
    – Yujie Zha
    Mar 20 at 1:26













    2












    $begingroup$

    You may assume (wlog) that $qgt 0$, so since $p^5/q^5=minBbb Z$, this implies $q^5mid p^5$. You can furthermore assume (wlog) that $gcd(p,q)=1$. This would imply that $qmid p$, so $1=gcd(p,q)=q$ but then $m=p^5$ leading to a contradiction that $m$ is a 5th power of an integer, namely $p$





    In fact, this argument can be generalized to $sqrt[n]m$ where $m$ is not a perfect $n^text{th}$ power






    share|cite|improve this answer









    $endgroup$


















      2












      $begingroup$

      You may assume (wlog) that $qgt 0$, so since $p^5/q^5=minBbb Z$, this implies $q^5mid p^5$. You can furthermore assume (wlog) that $gcd(p,q)=1$. This would imply that $qmid p$, so $1=gcd(p,q)=q$ but then $m=p^5$ leading to a contradiction that $m$ is a 5th power of an integer, namely $p$





      In fact, this argument can be generalized to $sqrt[n]m$ where $m$ is not a perfect $n^text{th}$ power






      share|cite|improve this answer









      $endgroup$
















        2












        2








        2





        $begingroup$

        You may assume (wlog) that $qgt 0$, so since $p^5/q^5=minBbb Z$, this implies $q^5mid p^5$. You can furthermore assume (wlog) that $gcd(p,q)=1$. This would imply that $qmid p$, so $1=gcd(p,q)=q$ but then $m=p^5$ leading to a contradiction that $m$ is a 5th power of an integer, namely $p$





        In fact, this argument can be generalized to $sqrt[n]m$ where $m$ is not a perfect $n^text{th}$ power






        share|cite|improve this answer









        $endgroup$



        You may assume (wlog) that $qgt 0$, so since $p^5/q^5=minBbb Z$, this implies $q^5mid p^5$. You can furthermore assume (wlog) that $gcd(p,q)=1$. This would imply that $qmid p$, so $1=gcd(p,q)=q$ but then $m=p^5$ leading to a contradiction that $m$ is a 5th power of an integer, namely $p$





        In fact, this argument can be generalized to $sqrt[n]m$ where $m$ is not a perfect $n^text{th}$ power







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Mar 16 at 5:37









        learnerlearner

        1,110417




        1,110417






























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