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Homework Help: Least squares fit to a straight line?

  1. Jan 30, 2010 #1
    I was wondering if someone could explain how to compute the Least squares fit to a straight line
  2. jcsd
  3. Jan 30, 2010 #2


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    Homework Helper

    You have N experimental points (xi,yi) and you want to fit a straight line y=ax+b across them so that the mean value of the square of the deviations y(xi)-yi is minimum with respect to the parameters a and b
    [tex]S=\sum_1^N{(ax_i+b-y_i)^2} = minimum[/tex]

    For that, the partial derivatives of S have to be zero. This condition yields two equations for a and b.

    [tex] \partial S /\partial a=\sum_1^N{2(ax_i+b-y_i)x_i}=0[/tex]
    [tex] \partial S /\partial b=\sum_1^N{2(ax_i+b-y_i)}=0[/tex]

    Rearranging the equations:

    [tex] a\sum_1^N{x_i^2}+b\sum_1^N{x_i}=\sum_1^N{x_i y_i}[/tex]

    [tex] a\sum_1^N{x_i}+N b=\sum_1^N{y_i}[/tex]

    Solve for a and b.

  4. Jan 30, 2010 #3
    i have solved it and got this

    y = 46.3x+(-61.8)

    my question is now, i plotted my points on a table on paper but how do i make the straight line?
  5. Jan 31, 2010 #4


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    Just calculate two points of your equation, put them onto the plot and connect them with a straight line :)

  6. Jan 31, 2010 #5
    how didnt i think of that duhhh lol, one more question now how do i calculate g from the slope?
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