Proving that, for $l$, $m$, $n$ odd integers, $sqrt{l^2-4mn}$ is never rational












0












$begingroup$


So the question I was solving was:



Prove that if $l$,$m$ & $n$ are odd then the line $lx+my+n=0$ will never intersect the parabola $y=x^2$ at a rational point.




I tried to solve the question and arrived at the condition that for this to be true $sqrt{l^2-4mn}$ should never be rational. How do I prove this?











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  • $begingroup$
    @Anguepa But then $l$ isn't odd.
    $endgroup$
    – saulspatz
    Nov 23 '18 at 16:59










  • $begingroup$
    You re right sorry. The square root of a natural number can only be an integer or irrational. Try to prove that the expression inside the square root can never be a perfect square.
    $endgroup$
    – Anguepa
    Nov 23 '18 at 17:07
















0












$begingroup$


So the question I was solving was:



Prove that if $l$,$m$ & $n$ are odd then the line $lx+my+n=0$ will never intersect the parabola $y=x^2$ at a rational point.




I tried to solve the question and arrived at the condition that for this to be true $sqrt{l^2-4mn}$ should never be rational. How do I prove this?











share|cite|improve this question











$endgroup$












  • $begingroup$
    @Anguepa But then $l$ isn't odd.
    $endgroup$
    – saulspatz
    Nov 23 '18 at 16:59










  • $begingroup$
    You re right sorry. The square root of a natural number can only be an integer or irrational. Try to prove that the expression inside the square root can never be a perfect square.
    $endgroup$
    – Anguepa
    Nov 23 '18 at 17:07














0












0








0


1



$begingroup$


So the question I was solving was:



Prove that if $l$,$m$ & $n$ are odd then the line $lx+my+n=0$ will never intersect the parabola $y=x^2$ at a rational point.




I tried to solve the question and arrived at the condition that for this to be true $sqrt{l^2-4mn}$ should never be rational. How do I prove this?











share|cite|improve this question











$endgroup$




So the question I was solving was:



Prove that if $l$,$m$ & $n$ are odd then the line $lx+my+n=0$ will never intersect the parabola $y=x^2$ at a rational point.




I tried to solve the question and arrived at the condition that for this to be true $sqrt{l^2-4mn}$ should never be rational. How do I prove this?








number-theory conic-sections






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edited Jan 3 at 18:56









Blue

49.3k870157




49.3k870157










asked Nov 23 '18 at 16:55









harshit54harshit54

346113




346113












  • $begingroup$
    @Anguepa But then $l$ isn't odd.
    $endgroup$
    – saulspatz
    Nov 23 '18 at 16:59










  • $begingroup$
    You re right sorry. The square root of a natural number can only be an integer or irrational. Try to prove that the expression inside the square root can never be a perfect square.
    $endgroup$
    – Anguepa
    Nov 23 '18 at 17:07


















  • $begingroup$
    @Anguepa But then $l$ isn't odd.
    $endgroup$
    – saulspatz
    Nov 23 '18 at 16:59










  • $begingroup$
    You re right sorry. The square root of a natural number can only be an integer or irrational. Try to prove that the expression inside the square root can never be a perfect square.
    $endgroup$
    – Anguepa
    Nov 23 '18 at 17:07
















$begingroup$
@Anguepa But then $l$ isn't odd.
$endgroup$
– saulspatz
Nov 23 '18 at 16:59




$begingroup$
@Anguepa But then $l$ isn't odd.
$endgroup$
– saulspatz
Nov 23 '18 at 16:59












$begingroup$
You re right sorry. The square root of a natural number can only be an integer or irrational. Try to prove that the expression inside the square root can never be a perfect square.
$endgroup$
– Anguepa
Nov 23 '18 at 17:07




$begingroup$
You re right sorry. The square root of a natural number can only be an integer or irrational. Try to prove that the expression inside the square root can never be a perfect square.
$endgroup$
– Anguepa
Nov 23 '18 at 17:07










2 Answers
2






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5












$begingroup$

Since $l^2-4mn$ would have to be an odd square, i.e. $1$ more than a multiple of $8$ just like $l^2$, $mn$ would be even, a contradiction.






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$endgroup$













  • $begingroup$
    Wow. Thanks for the quick response.
    $endgroup$
    – harshit54
    Nov 23 '18 at 17:05



















0












$begingroup$

The system of equations
$$begin{cases} my + lx + n = 0 \ y = x^2 end{cases}$$
Can be simplified as
$$mx^2 + lx + n = 0$$



As the right hand side is divisible by 2, the left hand side must be too.



But
$$mx^2 + lx + n equiv x^2 + x + 1 equiv x + x + 1 equiv 1 pmod{2}$$ as $m,l,n$ are odd.






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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    5












    $begingroup$

    Since $l^2-4mn$ would have to be an odd square, i.e. $1$ more than a multiple of $8$ just like $l^2$, $mn$ would be even, a contradiction.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Wow. Thanks for the quick response.
      $endgroup$
      – harshit54
      Nov 23 '18 at 17:05
















    5












    $begingroup$

    Since $l^2-4mn$ would have to be an odd square, i.e. $1$ more than a multiple of $8$ just like $l^2$, $mn$ would be even, a contradiction.






    share|cite|improve this answer









    $endgroup$













    • $begingroup$
      Wow. Thanks for the quick response.
      $endgroup$
      – harshit54
      Nov 23 '18 at 17:05














    5












    5








    5





    $begingroup$

    Since $l^2-4mn$ would have to be an odd square, i.e. $1$ more than a multiple of $8$ just like $l^2$, $mn$ would be even, a contradiction.






    share|cite|improve this answer









    $endgroup$



    Since $l^2-4mn$ would have to be an odd square, i.e. $1$ more than a multiple of $8$ just like $l^2$, $mn$ would be even, a contradiction.







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered Nov 23 '18 at 17:00









    J.G.J.G.

    32.6k23250




    32.6k23250












    • $begingroup$
      Wow. Thanks for the quick response.
      $endgroup$
      – harshit54
      Nov 23 '18 at 17:05


















    • $begingroup$
      Wow. Thanks for the quick response.
      $endgroup$
      – harshit54
      Nov 23 '18 at 17:05
















    $begingroup$
    Wow. Thanks for the quick response.
    $endgroup$
    – harshit54
    Nov 23 '18 at 17:05




    $begingroup$
    Wow. Thanks for the quick response.
    $endgroup$
    – harshit54
    Nov 23 '18 at 17:05











    0












    $begingroup$

    The system of equations
    $$begin{cases} my + lx + n = 0 \ y = x^2 end{cases}$$
    Can be simplified as
    $$mx^2 + lx + n = 0$$



    As the right hand side is divisible by 2, the left hand side must be too.



    But
    $$mx^2 + lx + n equiv x^2 + x + 1 equiv x + x + 1 equiv 1 pmod{2}$$ as $m,l,n$ are odd.






    share|cite|improve this answer









    $endgroup$


















      0












      $begingroup$

      The system of equations
      $$begin{cases} my + lx + n = 0 \ y = x^2 end{cases}$$
      Can be simplified as
      $$mx^2 + lx + n = 0$$



      As the right hand side is divisible by 2, the left hand side must be too.



      But
      $$mx^2 + lx + n equiv x^2 + x + 1 equiv x + x + 1 equiv 1 pmod{2}$$ as $m,l,n$ are odd.






      share|cite|improve this answer









      $endgroup$
















        0












        0








        0





        $begingroup$

        The system of equations
        $$begin{cases} my + lx + n = 0 \ y = x^2 end{cases}$$
        Can be simplified as
        $$mx^2 + lx + n = 0$$



        As the right hand side is divisible by 2, the left hand side must be too.



        But
        $$mx^2 + lx + n equiv x^2 + x + 1 equiv x + x + 1 equiv 1 pmod{2}$$ as $m,l,n$ are odd.






        share|cite|improve this answer









        $endgroup$



        The system of equations
        $$begin{cases} my + lx + n = 0 \ y = x^2 end{cases}$$
        Can be simplified as
        $$mx^2 + lx + n = 0$$



        As the right hand side is divisible by 2, the left hand side must be too.



        But
        $$mx^2 + lx + n equiv x^2 + x + 1 equiv x + x + 1 equiv 1 pmod{2}$$ as $m,l,n$ are odd.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Jan 3 at 19:10









        Jonas De SchouwerJonas De Schouwer

        3769




        3769






























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