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Irrational equations

  1. Feb 10, 2009 #1
    Show that the equation 4x^(2) + 5y^(2) = 2 has no rational solutions.

    Can this be done graphically?
  2. jcsd
  3. Feb 10, 2009 #2


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    If you have a rigorous graphical method, then of course. Though, I'm hard pressed to think of any that would be applicable....
  4. Feb 10, 2009 #3


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    Suppose the equation has rational roots:
    4x^(2) + 5y^(2) = 2
    then the question becomes does the equation:
    where a,b,c are natural numbers has solution.
    because 2c is even b must be even also, but then b=(4a-2c)/5 then 4a-2c is divisble by 5, i.e 4a-2c=0 or a and c are multiples of 5.
    if b=0 then c=2a i.e (q2q1)^2=2(p1q2)^2 but this isn't possible cause sqrt(2) is irrational.
    Now you need to check the option of multiples of 5 and get a contradiction, perhaps someone else with more intel in number theory will chip in with this question.
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