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Thread: Algebra help neede

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    Default Algebra help neede

    Is there someone who could help me re-arrange this Formula please?

    The Formula is

    a = 0.15 * b * b + b

    I need to calculate "b" in terms of "a" so I can place the Formula into a cell in Excel Program.



Look Here ->
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    Its a long time ago since I did quadratic equations but if my memory serves me right the solution is

    b= (-1 +/- (1+0.6a)^1/2)/0.3

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    a = 0.15b * b + b

    a = 0.15b2 + b

    a = b + 0.15b2

    a + -1b + -0.15b2 = b + -1b + 0.15b2 + -0.15b2

    b + -1b = 0

    a + -1b + -0.15b2 = 0 + 0.15b2 + -0.15b2

    a + -1b + -0.15b2 = 0.15b2 + -0.15b2

    0.15b2 + -0.15b2 = 0.00

    a + -1b + -0.15b2 = 0.00


    Now I'm confused!

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    Quote Originally Posted by verad View Post
    Its a long time ago since I did quadratic equations but if my memory serves me right the solution is

    b= (-1 +/- (1+0.6a)^1/2)/0.3
    In my Formula , I substituted 1.5 for b , and that gives a = 1.8375

    Therefore, if I substitute 1.8375 for a in your Formula Verad, then b should come up as 1.5

    0.6 * 1.8375 = 1.1025
    1 + 1.1025 = 2.1025
    2.1025^1/2 = 1.45
    -1 + 1.45 = .45
    .45/.3 = 1.5



    Thank You very much Verad.

    And thank you for your efforts as well IntelliGeorge.

    I don't suppose you could explain how you got the Formula Verad, in a similar way as how Intelligeorge tried to show?
    Not important as I have what I needed, but am curious, and Giving the Method is much more beneficial to the pupil than giving only the amswer.
    If you are busy I completely understand and very much appreciate what you have given regardless.
    Last edited by moeee; 11-04-08 at 07:47 PM. Reason: Added a Question

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    so how do you get " +/- " into a spreadsheet? Don't work on mine ? Using two columns for the formula ,one with + and the other with - I get different results, strange ? a with + b a with - (-1 + (1+0.6* B4)^1/2)/0.3 (-1 - (1+0.6* B4)^1/2)/0.3 4.33333 16.00000 -21.00000 8.33333 10.00000 -21.00000 9.33333 11.00000 -21.00000 369.33333 371.00000 -21.00000 -0.16667 1.50000 -21.00000 0.17083 1.83750 -21.00000 -1.66667 0 -21.00000 -1.66667 0 -21.00000

    I gave up tryying to format this , its a joke
    Last edited by tytower; 13-04-08 at 11:11 AM.

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    Quote Originally Posted by tytower View Post
    so how do you get " +/- " into a spreadsheet? Don't work on mine ?
    Probably won't work on mine either, but thats not what I put in mine.
    I just used the PLUS.
    It works fine and gives the correct answer.
    If instead I use the Minus, the answer comes out wrong.

    Verad was going a little on memory and perhaps his memory was almost perfect except for this little bit.
    I'm happy...and very grateful.

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    the formula verad gave is used to solve quadratic equations, ie your equation - of the form

    y = ax^2 + bx + c ------or in your case-------- a = 0.15b^2 + b

    I have attached a graph of your equation here:



    the roots of the equation are where the line crosses the horizontal axis - what you are looking for is the same thing essentially, just shifted up above / below the horizontal axis depending upon your value of a

    so, the +/- is supposed to be there, its just that it is not always a useful solution

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    Quadratic equations have two solutions so the + and - is supposed to be there.
    To go through the derivation of the solution is a bit complicated but if you do a google search on solution to quadratic equation the 4th item explains it.

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