How do I find points on a curve between known points?

[Guest]

I have a Logarithmic curve with points that do not match my Y-axis values.
How can I find the points that intercept my Y-axis major grid lines? I can
extrapolate the curve easily, but I cannot figure how to find other points on
the curve.

 

[Bernard Liengme]

First get the equation of the trendline into cells but not by manually
copying it. See
http://www.tushar-mehta.com/excel/tips/trendline_coefficients.htm
http://people.stfx.ca/bliengme/ExcelTips/Polynomial.htm
Then do some algebra.

What is the equation of the trendline? Suppose it is y=aLn(x)+b.
Do you want the value of y when x = 2? y= a*Ln(2)+b - use a spreadsheet!
Or do you want the value of x when y = 5? ln(x) = 5/a -b. Let this be c
So Ln(x) = c , then x = EXP(c)

best wishes