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Maths Problem

  • 25-10-2013 2:11pm
    #1
    MathsNerd31
    Registered Users, Registered Users 2 Posts: 17 ✭


    Can anyone solve this for me? It's too late on a Friday. I'm trying to get it using logs but keep backing myself into a corner.

    (0.5)^x=-3x-1


Comments

  • #2
    ZorbaTehZ
    Registered Users, Registered Users 2 Posts: 2,149 ✭✭✭ZorbaTehZ


    Is this from homework? Are you allowed to express the answer in terms of other functions? Because I think this is a transcendental equation so it does not have a closed-form solution...


  • #3
    MathsNerd31
    Registered Users, Registered Users 2 Posts: 17 ✭MathsNerd31


    Not for homework. A teacher friend of mine is looking for a solution and foolishly thought I'd be able to help. I think it was thrown at him by a student. A solution to it is x=-1 but not sure if there are others.


  • #4
    SHANAbert
    Registered Users, Registered Users 2 Posts: 162 ✭✭SHANAbert


    x^2 + 6x + 2 = 0

    x = -1 is not a solution.

    Work out the -b formula


  • #5
    Pembily
    Registered Users, Registered Users 2 Posts: 14,404 ✭✭✭✭Pembily


    Mod note: no solutions please, just advice.


  • #6
    greedygoblin
    Registered Users, Registered Users 2 Posts: 1,654 ✭✭✭greedygoblin


    MathsNerd31 wrote: »
    Not for homework. A teacher friend of mine is looking for a solution and foolishly thought I'd be able to help. I think it was thrown at him by a student. A solution to it is x=-1 but not sure if there are others.

    Let [latex] f(x) = 0.5^x [/latex]and plot.
    Let [latex] f(x) = -3x-1 [/latex] and plot.

    The intersection of the two curves should give you your root(s)...


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  • #7
    ZorbaTehZ
    Registered Users, Registered Users 2 Posts: 2,149 ✭✭✭ZorbaTehZ


    MathsNerd31 wrote: »
    Not for homework. A teacher friend of mine is looking for a solution and foolishly thought I'd be able to help. I think it was thrown at him by a student. A solution to it is x=-1 but not sure if there are others.

    There are exactly 2 which should be fairly easy to see if you plot both sides - the other one is only a little smaller than -1.
    SHANAbert wrote: »
    x^2 + 6x + 2 = 0

    x = -1 is not a solution.

    Work out the -b formula

    Er x=-1 is a solution, but neither root of your quadratic is a solution to the OPs equation.


  • #8
    Ciaran
    Registered Users, Registered Users 2 Posts: 939 ✭✭✭Ciaran


    SHANAbert wrote: »
    x^2 + 6x + 2 = 0

    x = -1 is not a solution.

    Work out the -b formula
    It's 0.5 to the power of x, not 0.5 by x squared.

    Graphing it is the quickest way to get a numerical solution. If the teacher wants the student to work it out by hand, they can use the Newton-Raphson method.


  • #9
    mathsnerd33
    Registered Users, Registered Users 2 Posts: 2 ✭mathsnerd33


    SHANAbert wrote: »
    x^2 + 6x + 2 = 0

    x = -1 is not a solution.

    Work out the -b formula

    Thanks but that's not the equation. Thanks everyone else too. I've got it now.


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