You can use the trendline feature to determine the formula. Select a
series, then use Add Trendline from the Chart menu.

To add these functions together, you can add together coefficients of
the individual trendlines. To avoid errors, display the trendline
formulas with a high degree of precision: format the formula, and on the
Number tab, choose a Scientific format with 15 decimal digits.

- Jon
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Jon Peltier, Microsoft Excel MVP
Peltier Technical Services
Tutorials and Custom Solutions
http://PeltierTech.com/
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Brian wrote:

I am trying to determine the equation for three different functions in a
scatter plot on graph in excel. I then want to have excel add these
functions together.

Does anyone know how to perform these functions?