Hugh  -

Looks like the Exponential type of trendline.  Here's a short answer:

You'll need more precision to get reliable results. So first select the
textbox on the chart, and press the Increase Decimal button numerous times
until 15 digits are displayed.

Then, on a worksheet, enter the 15-digit values, like this:

=612345.123456789*EXP(-5.10512345678901*2)

Here's some longer answers:

The Exponential type of trendline uses linear regression of log Y as the
dependent variable and X as the explanatory variable. You can obtain the
regression coefficients using the worksheet function LOGEST. And you can
obtain fitted values (e.g., for X = 2) using the GROWTH worksheet function.
See Excel's built-in Help for details, or see Chapter 15, "Simple Nonlinear
Regression," in my book "Data Analysis Using Microsoft Excel: Updated for
Office XP."

-  Mike

www.mikemiddleton.com

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