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Math Help - multiplication rules

  1. #1
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    multiplication rules

    This answer is obvious, but I can't formally justify why..

    Show if  a,b > 0 then  a\cdot b > 0 .
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  2. #2
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    If you think of multiplication as areas in the plane, two positive numbers multiplied together gives a rectangle in the first (positive) quadrant.
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  3. #3
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    What "postulates" or previous theorems do you have to work with?

    If you can use "if a> 0 and b> c then ab> ac" it is easy.

    If not, perhaps you have, as part of the definition of ">", the "trichotomy" postulate:
    If a and b are any two numbers, then one and only one of these is true:
    1) a= b
    2) a> b
    3) b> a

    If not either of those, what definition of ">" are you using?
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