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Math Help - Unusual case of log - log plot

  1. #1
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    Unusual case of log - log plot

    If y = ax^n then on a log x; log y plot (Eg in Excel) the gradient of the resulting straight line is n as logy = loga + nlogX. But in our particular problem y=a(x-b)^n (where b is a constant) and we do not know how to work out the new gradient on a logx ; logy plot despite many attempts with differential methods. By plotting examples in Excel it looks like the gradient has been modified to nx/(x-b). We would really like to show the working to prove this or otherwise. As we would like to include this solution in a publication we would happily acknowledge the author / source.
    Thanks in advance,
    Chris
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by Chrisgarrard View Post
    If y = ax^n then on a log x; log y plot (Eg in Excel) the gradient of the resulting straight line is n as logy = loga + nlogX. But in our particular problem y=a(x-b)^n (where b is a constant) and we do not know how to work out the new gradient on a logx ; logy plot despite many attempts with differential methods. By plotting examples in Excel it looks like the gradient has been modified to nx/(x-b). We would really like to show the working to prove this or otherwise. As we would like to include this solution in a publication we would happily acknowledge the author / source.
    Thanks in advance,
    Chris
    Tell us what you are raely trying to do (like say estimate a, b and n from data).

    CB
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