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Homework Help: Elementary math proof

  1. Apr 30, 2016 #1
    1. The problem statement, all variables and given/known data
    Proof 2/5*(2^0.5)-1/7 is irrational

    2. Relevant equations

    3. The attempt at a solution
    I did this by splitting the expression and setting contradictions

    Proof first rational times irrational is irrational

    Proof by contradiction

    Assume the product is rational
    let rational be x/y irrational s and the product u/t


    Contradiction s can't be rational

    and then I do the same thing for irrational-rational

    Is that the right approach?
  2. jcsd
  3. Apr 30, 2016 #2

    Ken G

    User Avatar
    Gold Member

    Sure-- a proof is a proof, and that would prove it. The question I have is if you are allowed to take as given that the square root of 2 is irrational, but if you are, proof by contradiction is certainly the way to go.
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