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Thread: Numerical differentiation problem?

  1. #1
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    Numerical differentiation problem?

    xi=[ 0.1 0.2 0.3 0.4 0.5 0.6]

    yi=[1.621 1.408 1.266 1.170 1.104 1.060]

    What is d ln(yi) / d ln(xi) = ?
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by chronicals View Post
    xi=[ 0.1 0.2 0.3 0.4 0.5 0.6]

    yi=[1.621 1.408 1.266 1.170 1.104 1.060]

    What is d ln(yi) / d ln(xi) = ?
    find the log of each of the xi's and yi's. then use the slope formula to find the derivative. remember, the derivative gives the slope.
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  3. #3
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    Quote Originally Posted by Jhevon View Post
    find the log of each of the xi's and yi's. then use the slope formula to find the derivative. remember, the derivative gives the slope.
    I write this in matlab:

    >> x=0.1:0.1:0.6

    x =

    0.1000 0.2000 0.3000 0.4000 0.5000 0.6000

    y=[1.621 1.408 1.266 1.170 1.104 1.060];

    >> k=log(y)

    k =

    0.4830 0.3422 0.2359 0.1570 0.0989 0.0583

    >> l=log(x)

    l =

    -2.3026 -1.6094 -1.2040 -0.9163 -0.6931 -0.5108

    and this our result:

    >> z=diff(k)./diff(l)

    z =

    -0.2032 -0.2622 -0.2741 -0.2602 -0.2231

    but in my book the result is different:

    d ln(y) / d ln (x)= [ -0.246 - -0.249 - -0.2.44 -]

    i must reach the book's result, but how?
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