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Math Help - multiplication proof

  1. #1
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    multiplication proof

    w > x and y > z then wy > xz

    w,x, y and z are natural numbers

    use only addition and multiplication for proof

    I need help with this proof ASAP....please
    Last edited by jay0148; December 4th 2007 at 05:27 PM.
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by jay0148 View Post
    w > x and y > z then wy > xz

    w,x, y and z are natural numbers

    I need help with this proof ASAP....please
    The trouble here is that this is almost too obvious. The conclusion seems to follow immediately for natural numbers. So to make our proof seem more rigorous, let's take the long way around.

    Proof

    Let w,x,y,z \in \mathbb{N}.

    Suppose w \ge x and y \ge z. Then we have that w - x \ge 0 and y - z \ge 0. We now have two numbers that are non-negative, therefore, their product must be non-negative. Thus we have:

    0 \le (w - x)(y - z) = wy - xz -xy - wz \Longleftrightarrow xz + (xy + wz) \le wy.

    Since w,x,y,z \in \mathbb{N}, xy + wz \ge 0. Thus we make the left side of the last inequality even smaller if we subtract it. Hence,

    wy \ge xz + (xy + wz) \ge xz \implies wy \ge xz as desired.

    QED
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  3. #3
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    Thanks!

    Thanks!!
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  4. #4
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    I would like to do this proof using only multiplication and addition, if anyone else can help...thanks.
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