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Math Help - Cubic Spline URGENT help!

  1. #1
    Junior Member
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    Oct 2010
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    Cubic Spline URGENT help!

    Here is my question: In order to evaluate for , you need


    I wrote a program to create a table of the coefficients however im lost on how to find the coef.

    Here is my table:
    Coefficients [x a b c d]
    0.9000 1.3000 0.5396 0 -0.2476
    1.3000 1.5000 0.4208 -0.2972 0.9469
    1.9000 1.8500 1.0868 1.4073 -2.9564
    2.1000 2.1000 1.2949 -0.3666 -0.4466
    2.6000 2.6000 0.5934 -1.0365 0.4451
    3.0000 2.7000 -0.0222 -0.5025 0.1742
    3.9000 2.4000 -0.5034 -0.0322 0.0781
    4.4000 2.1500 -0.4771 0.0849 1.3142
    4.7000 2.0500 -0.0713 1.2676 -1.5812
    5.0000 2.1000 0.2623 -0.1555 0.0431
    6.0000 2.2500 0.0808 -0.0261 -0.0047
    7.0000 2.3000 0.0146 -0.0401 -0.0244
    8.0000 2.2500 -0.1390 -0.1135 0.0175
    9.2000 1.9500 -0.3358 -0.0506 -0.0127
    10.5000 1.4000 -0.5318 -0.1002 -0.0203
    11.3000 0.9000 -0.7312 -0.1490 1.2134
    11.6000 0.7000 -0.4929 0.9431 -0.8393
    12.0000 0.6000 -0.1413 -0.0640 0.0364
    12.6000 0.5000 -0.1789 0.0014 -0.4480
    13.0000 0.4000 -0.3928 -0.5361 0.5957
    13.3000 0.2500 0 0 0
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  2. #2
    Junior Member
    Joined
    Oct 2010
    Posts
    27

    Better table

    Cubic Spline URGENT help!-untitled-1.png


    Ok i figured it out. Its actually pretty simple. so lets say you want to solve S(x) when x=11.8
    what you do is u go to the table and look for where 11.8 is in between two values.
    x a b c d
    11.6000 0.7000 -0.4929 0.9431 -0.8393
    12.0000 0.6000 -0.1413 -0.0640 0.0364

    as you can see 11.8 fits between these two rows SOOO you take the values of the smaller row.
    11.6000 0.7000 -0.4929 0.9431 -0.8393

    and create the following equation for S(x)
    S(x) = -.8383*(x-11.6)^3+.943*(x-11.6)^2-.4929*(x-11.6)+.7000

    then plug in the value u are solving for x=11.8
    S(11.8) = -.8383*(11.8-11.6)^3+.943*(11.8-11.6)^2-.4929*11.8+6.41764
    Last edited by ductiletoaster; October 24th 2010 at 03:37 PM.
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