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Math Help - Finding an unknown in a formula

  1. #1
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    Finding an unknown in a formula

    1 / 4/5 + 1/v = 1/f

    Find the value of v

    1 / 4/5 + 1/v = 1 / 2/3 - 1 / 4/5 = 1/v

    1 / 2/3 - 1 / 4/5 = 5/3 - 1 x 9 / 1 x 5 = 5/3 - 9/5 =

    5 x 5 / 3 x 9 = 25 / 27

    I am sure I am not quite right with this?
    Last edited by David Green; November 18th 2011 at 03:26 AM. Reason: Can't get latex working correctly
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  2. #2
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    Re: Finding an unknown in a formula

    By 1 / 4/5 do you mean (a) 1\tfrac{4}{5}, (b) \frac{1}{4/5} or (c) \frac{1/4}{5}? These options can be rendered in text as follows: (a) 1 4/5, (b) 1 / (4 / 5) and (c) (1 / 4) / 5 (maybe also adding a note would help).

    Your equation has two variables: v and f, so at most you can express v through f.

    LaTeX formulas have to be surrounded by [TEX]...[/TEX] tags (that's the rightmost button over the text area where you type your post, after you press "Preview Post" button).
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  3. #3
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    Re: Finding an unknown in a formula

    Quote Originally Posted by emakarov View Post
    By 1 / 4/5 do you mean (a) 1\tfrac{4}{5}, (b) \frac{1}{4/5} or (c) \frac{1/4}{5}? These options can be rendered in text as follows: (a) 1 4/5, (b) 1 / (4 / 5) and (c) (1 / 4) / 5 (maybe also adding a note would help).

    Your equation has two variables: v and f, so at most you can express v through f.

    LaTeX formulas have to be surrounded by [TEX]...[/TEX] tags (that's the rightmost button over the text area where you type your post, after you press "Preview Post" button).
    I mean what you said in (b) above.

    Thanks for the additional help with latex, I am sure I get it right soon.

    David
    Last edited by David Green; November 18th 2011 at 03:48 AM. Reason: typo error
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  4. #4
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    Re: Finding an unknown in a formula

    Since \frac{1}{4/5}=\frac{5}{4}, the equation is \frac{5}{4}+\frac{1}{v}=\frac{1}{f}.

    \frac{1}{v}=\frac{1}{f}-\frac{5}{4}=\frac{4-5f}{4f}
    v=\frac{4f}{4-5f}
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  5. #5
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    Re: Finding an unknown in a formula

    Quote Originally Posted by emakarov View Post
    Since \frac{1}{4/5}=\frac{5}{4}, the equation is \frac{5}{4}+\frac{1}{v}=\frac{1}{f}.

    \frac{1}{v}=\frac{1}{f}-\frac{5}{4}=\frac{4-5f}{4f}
    v=\frac{4f}{4-5f}
    Thanks for your help we have now solved it.

    P.S. your PM box is full lol

    David
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