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Math Help - 3D geometry with quadriatic problem need real help!!!

  1. #1
    Junior Member
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    Mar 2009
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    Exclamation standard deviation need help!!!!

    IM using my brothers account and so Im kinda stuck on these questions thanks
    Attached Thumbnails Attached Thumbnails 3D  geometry with quadriatic problem need real help!!!-scan0036.jpg  
    Last edited by osmosis786; April 13th 2009 at 09:49 AM.
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  2. #2
    Member
    Joined
    Nov 2008
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    Hi there,

    Just work through it step by step, each time subbing what you know

    So we know that

    L=\frac{1}{3}(E-24x)

    we're told what E is so we can now write.

    L=\frac{1}{3}(98-24x) = \frac{98}{3}-8x

    Now we can put this value of L into our formula for A, which gives us

    448 = 12x^2 +12\big(\frac{98}{3} - 8x \big)x
    448 = 12x^2  + 392x - 96x^2

    Rearrange a bit

    84x^2 - 392x + 448= 0

    All the terms have a common factor of 28, so we can divide through to get

    3x^2 - 14x + 16 = 0

    as required
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  3. #3
    Junior Member
    Joined
    Mar 2009
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    thanks

    that was knida what I was doing in the first place but instead of leaving the 98/3 as a fraction I left it as a decimal so i thnk thats where I went rong... anyway thanks +rep
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  4. #4
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    Fractions are much nicer to work with algebraically, so keep everything as a fraction for as long as you can
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