Very tricky maths question... please help

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  1. bluebird13's Avatar
    1 05-07-2012  02:00
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    Very tricky maths question... please help
    I would really appreciate it if someone could help me with question 6 of this past exam paper... the answer is A but the answer I am getting is slightly different and i'm not sure why....

    Thank you so much in advance... its been making my brain hurt for a while now!


    http://www.admissionstests.cambridge..._Section_2.pdf
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  2. Lord of the Flies's Avatar
    2 05-07-2012  12:40
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    • Location: Paris, France
    Re: Very tricky maths question... please help
    t=2\pi\sqrt{\dfrac{2 l R^2(W+w/3)}{n\pi r^4 g}}

    t^2=4\pi^2 \dfrac{2 l R^2(W+w/3)}{n\pi r^4 g}

    t^2=\dfrac{8\pi l R^2(W+w/3)}{nr^4 g}

    t^2(nr^4 g)=8\pi l R^2(W+w/3)

    \dfrac{t^2(nr^4 g)}{8\pi l R^2}=W+w/3

    W=\dfrac{t^2(nr^4 g)}{8\pi l R^2}-w/3
  3. Lord of the Flies's Avatar
    3 05-07-2012  12:51
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    • Location: Paris, France
    Re: Very tricky maths question... please help
    (Original post by bluebird13)
    ...
    On a side note, that's a lot of questions to do in 30 min! :eek:
  4. bluebird13's Avatar
    4 05-07-2012  15:22
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    Re: Very tricky maths question... please help
    (Original post by Lord of the Flies)
    On a side note, that's a lot of questions to do in 30 min! :eek:
    Thank you very much! you star! and yes roughly a minute to read the question, work out what do to and write it all down seems pretty mad!
  5. ghostwalker's Avatar
    5 05-07-2012  16:32
    • Outcast of Imrryr
    • Location: CA13
    Re: Very tricky maths question... please help
    As far as the exam goes, if you actually work through rearranging the formula, it's going to take longer than necessary.

    Looking at the original equation, your penultimate step is going to have W+w/3 by itself, so you're going to end up with something -w/3.

    That eliminates C, and D.

    Then if you look what happens to pi; you're squaring the eqn, and you have pi squared over pi, leaving you with just pi. It flips to the denominator when you move it over, although more obviously, there is no root(pi), hence B is eliminated, leaving A.
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