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Math Help - Proving an Equation...

  1. #1
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    Proving an Equation...

    Did I do the math right? Because the answer doesn't make any sense.


    http://i.imgur.com/P97Gs.png
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  2. #2
    Member kalyanram's Avatar
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    Re: Proving an Equation...

    The math is wrong. Check step 3. How did you get it from step 2?
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  3. #3
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    Re: Proving an Equation...

    I flipped (1/x) and (1/y). I can't do that without flipping the other side, correct?
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  4. #4
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    Re: Proving an Equation...

    So would this be right...?

    http://i.imgur.com/XNNhI.png

    Where would I go from here?
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  5. #5
    Senior Member MaxJasper's Avatar
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    Lightbulb Re: Proving an Equation...

    Quote Originally Posted by johnhisenburg87 View Post
    Did I do the math right? Because the answer doesn't make any sense.
    http://i.imgur.com/P97Gs.png
    \frac{2 x y}{x+y}\leq \sqrt{x y}

    \frac{\sqrt{x y}}{x+y}\leq \frac{1}{2}

    (x+y)^2 \geq 4x y

    x^2+y^2+2x y \geq 4 x y

    x^2+y^2-2x y\geq 0

    (x-y)^2 \geq 0

    Inequality is true if x \geq y

    Because x & y are interchangeable in the original inequality relation then we also conclude that:

    Inequality is true if y \geq x

    Hence: the only valid solution is x=y

    This means inequality is not true for any x\neq y, i.e., only euality portion is true for x=y

    ...check this out...

    Thanks to Bingk for remarks.
    Last edited by MaxJasper; September 11th 2012 at 02:56 PM.
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  6. #6
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    Re: Proving an Equation...

    Just some things to note, you're kinda working backwards. You should start with a statement that you knows is true, then manipulate it to get the conclusion.

    Try starting with this:

    (\sqrt{x} - \sqrt{y})^2 \geq 0.

    You're allowed to take the square roots because the problem states that x and y are positive real numbers.
    Also, note that even if x is less than y, the difference is squared, so the resulting values will always be nonnegative, that's why you know your starting statement is true.


    Last thing, on the post above by this by MaxJasper, near the end it says
    Inequality is true if x>y
    this is partially true, the inequality is also true if x<y because the difference is squared
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