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Math Help - Making f the subject

  1. #1
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    Making f the subject

    Making f the subject of:

    Sqrt(f+p)/sqrt(f−p)=D/d

    my first step was to square, so I had:

    (F+p)/(f-p) = ( D/d)^2

    then multiply by (f-p)

    F+p = (f-p)(D/d)^2

    now I get stuck because if I now want to remove f from one side I have to subtract f but that will leave me without any f's, please help!

    Thanks

    kris
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  2. #2
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    Re: Making f the subject

    My apologise also for not writing in latex but I'm new and still trying to get to grips with it.
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  3. #3
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    Re: Making f the subject

    f+p = (f-p)\left(\frac{D}{d}\right)^2

    f+p = f\left(\frac{D}{d}\right)^2 - p\left(\frac{D}{d}\right)^2

    p + p\left(\frac{D}{d}\right)^2 = f\left(\frac{D}{d}\right)^2 - f

    p\left[1 + \left(\frac{D}{d}\right)^2\right] = f \left[\left(\frac{D}{d}\right)^2 - 1\right]

    \frac{p\left[1 + \left(\frac{D}{d}\right)^2\right]}{\left[\left(\frac{D}{d}\right)^2 - 1\right]} = f
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  4. #4
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    Re: Making f the subject

    Wow I don't understand how the (D/d)^2 gets on both sides and also how the f and -f don't cancel. Awesome job though, just wish I had your transposing skills,
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