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Sine Regression

  1. Aug 28, 2011 #1
    When I try and model a curve using a sine regression with my graphic calculator, it returns an error message that says: Singular Matrix

    I decided then to do it by hand. Here are three points.

    Minimum: (0,554.8), Maximum: (45, 1220.5) and a random point (20, 830.7)

    I want something in the format of: A*sin B(x-c) + d

    This is what I did, is it correct?

    A = (1220.5-554.8)/2
    B = (2*pi)/45
    D = (1220.5+554.8)/2
    C = One unknown, so I used the random point shown to solve:

    (Should I have calc in radians or degrees?)

    My steps: ( I will use letters for simplicity)

    830.7 = A*sinB(x-c)+d
    830.7 - D = A*sinB(x-c)
    Ans/A = sinB(x-c)
    sin-1(Ans/A) = B(x-c)
    Ans/B=x-c
    -c=(Ans/B)-x

    But I am not getting anything sensible...

    Is my calculator right or did I do anything wrong?

    Thanks
     
  2. jcsd
  3. Aug 29, 2011 #2

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Unless a problem specifically gives a value as the measure of an angle in degrees, you should always use radians. Unfortunately, you don't say how the given information was derived. What are the units for the x variablle? That "45" makes me wonder if it not in degrees. The distance from max to min for a sine function is [itex]\pi[/itex] radians or 180 degrees, NOT [itex]2\pi[/itex] radians or 360 degrees.

    YOur formulas for A and D are correct.

    30,7
     
  4. Aug 29, 2011 #3
    So I should do pi over 45 years instead of 2pi over 45?
     
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