Excel 2003 doesn't display the proper equation with trendlines

[wsander2]

Excel 2003 is not displaying the proper equation if you insert a trendline to
data and click on the "display equation" check box. If you use the equation
to generate "y" from the exact "x" values that you began with, you get a
completely different curve than the original trendline displayed on the
graph. For example: Use the "log" function in a worksheet to plot y=log(x)
from x=1 to 100, plot the data, and fit a 2nd order polynomial to this data,
clicking on "display equation". It won't be a good fit, but this is beside
the point. Use the equation Excel generated to re-plot the y=f(x) values x=1
to 100. The data does not fall on the trend line. Why is this?

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[wsander2]

I should add that I've tried to increase the significant digits in the
equation to 30 in scientific, but Excel doesn't appear to be responding to
this. I still get an equation that looks like "y = -0.0002x2 + 0.0373x +
0.4745"

 

[Jerry W. Lewis]

When I reformat that trendline equation to show more figures, I get
y = -1.94230739776658E-04*x^2 + 3.20631890271028E-02*x + 6.17688698712797E-01
in both Excel 2003 and Excel 2000.

Jerry