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Homework Help: My proof involving Pythagorean’s Theorem

  1. Feb 27, 2010 #1
    1. The problem statement, all variables and given/known data

    Let a, b, and c be lengths of sides of triangle T, where a ≤ b ≤ c.
    Prove that if T is a right triangle, then (abc)2=(c6-a6-b6)/3

    2. Relevant equations

    If T is a right triangle, then Pythagorean’s Theorem states:
    The sum of the squares of the lengths of the sides of a right triangle is equal to the square of the length of the hypotenuse. That is a2+b2=c2, where c is the hypotenuse.

    3. The attempt at a solution

    We assume the given equation and using Pythagorean’s Theorem, we obtain solutions for c2 and c6:
    We substitute these results into the original equation.
    This produces an equation where the left hand side is identical to the right hand side.
    Since these terms are equal, it follows that the original equation holds true for a right triangle.

    This is what I have. I am curious to if the proof is correct/acceptable.

    Thanks for any feedback.
  2. jcsd
  3. Feb 27, 2010 #2
    Re: My proof involving Pythagorean’s Theorem

    Can you be more specific? It sounds like you're plugging in what you find into the identity you are trying to prove, which is flawed in that it assumes your identity is true in the first place. You want to start with the expression on one side of the identity and get to the other expression. (You keep referring to the "original equation", which I take to be the identity you are trying to prove, and not the pythagorean identity).

    For instance, take the pythagorean identity and raise both sides to the third power. Subtract a^6 and b^6 from both sides to get an expression for c^6 - a^6 -b^6. Divide by 3, factor, and use the pythagorean identity again to get the left hand side of the identity you are trying to prove.
  4. Feb 27, 2010 #3


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    You didn't really show what you did. But if you substituted c=sqrt(a^2+b^2) and got the same result on both sides, I think that's just fine.
  5. Feb 27, 2010 #4
    Thanks for the help snipez90 and Dick. I redid the problem.

    This is my original:

    And this is my new proof:

    Attached Files:

  6. Feb 27, 2010 #5


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    Gold Member

    Re: My proof involving Pythagorean’s Theorem

    Your first proof wasn't really a proof at all since you assumed what you wanted to prove and then arrived at a familiar equality (although, this can be a useful strategy if every step that you take is reversible). Your second proof looks correct to me however.
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