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Finding X-axis Intercept

  1. #1
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    Finding X-axis Intercept

    So I've got some data, which has the approximate form of a sine function. I want to find all the x-axis intercepts. I tried using the intercept function and swapping around the y values for the x values, but it only returns 1 value (so I'd guess it uses a linear regression to estimate a single line through the axis).

    I was thinking of trying a nested if/and statement but I haven't quite figured out how to do it. Basically I want to identify the two values where it switches from positive to negative and also indentify the values where it goes negative to positive, I can then fit a straight line between them to find a better approximation of the intercept (though it might not be necessary). Preferably I'd like it all one function as I'm not doing it in VBA (I might do later though, we'll see).

    Can anyone suggest how I'd find these value or the x-intercept. Any help would be greatly appreciated.

  2. #2
    Forum Guru shg's Avatar
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    Re: Finding X-axis Intercept

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    The formula in D2 and down is

    =IF(SIGN(B2) = SIGN(B3), NA(), A2 + (A3-A2) * B2/(B2-B3)
    Last edited by shg; 08-23-2010 at 06:27 PM.
    Entia non sunt multiplicanda sine necessitate

  3. #3
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    Re: Finding X-axis Intercept

    Thanks, it worked perfectly, I did not know about the sign function.

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