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Homework Help: Simple equation

  1. Mar 12, 2009 #1
    How can I solve this equation:

    \frac{1}{d^2} = \frac{1}{(9m)^2} + \frac{1}{(5m)^2}



    \frac{1}{d^2} = \frac{1}{81m^2} + \frac{1}{25m^2} \Rightarrow d^2 = 81m^2 + 25m^2 = 106m^2 \Rightarrow d = 10.3m
  2. jcsd
  3. Mar 12, 2009 #2


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    Science Advisor

    up to here you are fine. But
    is not true. if
    [tex]\frac{1}{a}= \frac{1}{b}+ \frac{1}{c}[/tex]
    it is NOT true that a= b+ c.
    More specifically,
    [tex]\frac{1}{\frac{1}{b}+ \frac{1}{c}}\ne b+ c[/tex]

    I recommend that you multiply both sides of the equation by the least common divisor of all three fractions, (25)(81)d2.
    Last edited by a moderator: Mar 12, 2009
  4. Mar 12, 2009 #3


    Staff: Mentor

    Or even better, by (25)(81)d2m2
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