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Math Help - How to rearrange to make D the subject?

  1. #1
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    How to rearrange to make D the subject?

    Hi,

    For some reason i'm struggling to rearrange a lot of equations where i'm attempting to make a variable that appears twice in the equation the subject. At the moment i am attempting to rearrange this equation to make D the subject. May someone please show me how to do this in steps please?

    A=B*D*(1/(C+D))


    And if possible, i would like some tips on how to make variables that appear twice the subject please?


    Any help is much appreciated
    Rob
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  2. #2
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    Re: How to rearrange to make D the subject?

    You have A=\frac{BD}{C+D}

    A good first step would be to multiply by the denominator to get

    A(C+D) = BD

    Expand the brackets

    AC+AD=BD

    Rearrange a little

    AC=BD-AD

    factorise, and I'll leave the rest to you.
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  3. #3
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    Re: How to rearrange to make D the subject?

    Hello, Rob!

    \text{Solve for }D\!:\;\;A \:=\:\frac{BD}{C+D}

    Since there is a D in the denominator, multiply by (C+D)

    . . A(C+D) \:=\:BD \quad\Rightarrow\quad AC + AD \:=\:BD


    Get all D-terms on one side, the rest on the other side.

    . . AD - BD \:=\:-AC


    Factor out the D.

    , . (A-B)D\:=\:-AC


    Solve for D . . . divide by (A-B)

    . . D \;=\;\frac{-AC}{A-B}


    ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~


    This answer is correct,
    . . but you may see it in a different form.

    Multiply the fraction by \frac{\text{-}1}{\text{-}1}

    . . D \;=\;\frac{\text{-}1}{\text{-}1}\cdot\frac{-AC}{A-B} \;=\;\frac{AC}{B-A}
    Thanks from Shocker
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  4. #4
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    Re: How to rearrange to make D the subject?

    Quote Originally Posted by a tutor View Post
    AC=BD-AD

    factorise, and I'll leave the rest to you.
    I managed to get it up to this point but for some reason i just couldn't think about how to do the next step. It was just so simple. I kind of kick myself for not being able to figure it out. lol



    Much appreciated to you both.
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