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Math Help - Algebra/Trig - HELP!!!!!

  1. #1
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    Algebra/Trig - HELP!!!!!

    Hey,
    I have an exam in two days!
    And I'm tearing my hair out (and so are all of my friends - I've asked 12 people this question and none of them know how to do it)...

    xsin(x)=5
    x=

    Must use algebraic method for working (not allowed to use graphs).

    Please help quickly!
    Thanks,
    Bobsalive
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by bobsalive View Post
    Hey,
    I have an exam in two days!
    And I'm tearing my hair out (and so are all of my friends - I've asked 12 people this question and none of them know how to do it)...

    xsin(x)=5
    x=

    Must use algebraic method for working (not allowed to use graphs).

    Please help quickly!
    Thanks,
    Bobsalive
    I'm not sure what you are talking about. This equation has no solution in terms of elementary functions. If you are allowed Calculus there are a variety of techniques you can use, but if you are denied that the only thing I can really think of is to do a "binary search" for the solution, which amounts to little more than guessing the solution and plugging numbers into a calculator. The method is not algebraic.

    -Dan
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  3. #3
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    Hey,
    Thanks for replying!
    We are allowed to use calculus (I think this might be a calculus question - they aren't specifically set out in sections or anything like that).

    In terms of the binary search thing, we use a graphical method (using GDCs), and just plugged in y=xsin(x) and y=5 and found the intercepts, which occured at +/- 17.05 (rounded to 4 sf). But we still don't know how to get to this answer via non-graphical means.

    Hopefully that helps with trying to answer the question! ANY leads are appreciated =)

    Thanks,
    Bobsalive

    (and just for fun, someone else asked us "xsin(x)=0 x=?")
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  4. #4
    Junior Member Spimon's Avatar
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    Quote Originally Posted by bobsalive View Post
    and just for fun, someone else asked us "xsin(x)=0" x=?
    Easy! Anything of which the sine function = 0. X = 0 or 180

    Sorry, can't help with the original question. Still thinking about it.
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  5. #5
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    Quote Originally Posted by topsquark View Post
    is to do a "binary search" for the solution, which amounts to little more than guessing the solution and plugging numbers into a calculator. The method is not algebraic.

    -Dan
    Hey,
    Sorry for misleading people in my first post - it doesn't have to be an algebraic method. It just has to be non-graphical and with working (so you can use calculus or trigonometry or whatever.)

    Thanks,
    Bobsalive
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  6. #6
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by bobsalive View Post
    Hey,
    Sorry for misleading people in my first post - it doesn't have to be an algebraic method. It just has to be non-graphical and with working (so you can use calculus or trigonometry or whatever.)

    Thanks,
    Bobsalive
    Try the Newton-Raphson method.

    -Dan
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  7. #7
    Senior Member DivideBy0's Avatar
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    There's a nice video on the Newton-Raphson method here. It's a link from this great calculus videos site to which the link was posted in "Useful Math Websites" by anyman.
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