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Thread: explain the fifth property of fractions

  1. #1
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    explain the fifth property of fractions

    property: if a/b = c/d then ad = bc
    description: cross-multiply

    can someone explain this to me with proof? i don't understand why it works.
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  2. #2
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    Re: explain the fifth property of fractions

    you should be able to see this by now...

    $\dfrac a b = \dfrac c d$

    $bd \dfrac a b = bd \dfrac c d$

    $d a = b c$

    $ad = bc$
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  3. #3
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    Re: explain the fifth property of fractions

    it's sinking in
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  4. #4
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    Re: explain the fifth property of fractions

    Or "step by step": From $\frac{a}{b}= \frac{c}{d}$
    multiply both sides by b: $\frac{a}{b}b= \frac{c}{d}b$ so $a= \frac{bc}{d}$

    multiply both sides by d: $ad= \frac{bc}{d}d$ so $ad= bc$.
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