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Finding cubic function

  1. Jan 17, 2008 #1
    1. The problem statement, all variables and given/known data

    X-intercepts: (-1.57,0) , (0.65, 0) , (2.83, 0)
    Y-intercept: 11.33



    2. Relevant equations

    I've got to convert that information into an estimated cubic function.


    3. The attempt at a solution

    I tried subbing the y-intercept in; although that didn't work.

    11.33 = K(1.57)(-0.65)(-2.83)
    11.33 = K(2.888)
    K = 3.923

    That didn't provide the correct equation when subbed into

    f(x)=K(x+1.57)(x-0.65)(x-2.83)

    Can anyone help me on this?
     
  2. jcsd
  3. Jan 17, 2008 #2

    symbolipoint

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    I checked your solution process and tried your results. No problem found. You only have four points to use and to check. They all work in your function which you found.
     
  4. Jan 18, 2008 #3

    HallsofIvy

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    Why do you say "that didn't provide the correct equation"?

    Certainly f(x)= 3.923(x+ 1.57)(x- 0.65)(x- 2.83) is 0 at x= -1.57, x= 0.65, and x= 2.83 and, as you calculated f(0)= 3.923(.157)(-0.65)(2.83)= 11.33. It's a cubic and it satisfies all the requirements.
     
  5. Jan 19, 2008 #4
    It was my own fault; I mixed up a few things when I was graphing the equation on a graphing calculator.

    Thanks for the help though.
     
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