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Rational expressions

  1. Jul 23, 2008 #1
    1. x/20 = (3/8)-(4/5)

    2. solve

    3. My attempt as far as I can tell there is no LCM so

    3/8 becomes 15/40 4/5 becomes 32/40

    (15/40)-(32/40)= 17/40 which equals 8.5/20 which means x=8.5

    For some reason I don't think I got the right answer???
  2. jcsd
  3. Jul 23, 2008 #2


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    [tex]\frac{15}{40}- \frac{32}{40} \neq \frac{17}{40}[/tex]

    15/40 is less than 32/40. Do you think you answer would be positive?

    If you have to find 3-4, you won't get 1. You'd get -1.
  4. Jul 23, 2008 #3
    o.k. how do you get a LCM to factor with if the only LCM is 1?

    Man I am confused :P
  5. Jul 23, 2008 #4


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    Let's start over then.

    [tex]\frac{3}{8}- \frac{4}{5}[/tex]

    We agree that the common denominator is 40,right?

    [tex]\frac{3}{8}- \frac{4}{5}= \frac{15}{40} - \frac{32}{40}[/tex]

    Understand up to here right?

    Now, if the denominators are the same, you can write it as one single fraction like so:

    [tex]\frac{15}{40} - \frac{32}{40}= \frac{15-32}{40}[/tex]

    Now you must find the value of 15-32
  6. Jul 23, 2008 #5
    so that would be -17

    -17/40 so it would be -8.5
  7. Jul 23, 2008 #6
    I guess what is confusing me is that the assignment was about solving equations after finding and factoring the LCM.
  8. Jul 23, 2008 #7


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