Returns the sum of a power series based on the formula:
Many functions can be approximated by a power series expansion.
If this function is not available, and returns the #NAME? error, install and load the Analysis ToolPak addin.
How?
 On the Tools menu, click AddIns.
 In the AddIns available list, select the Analysis ToolPak box, and then click OK.
 If necessary, follow the instructions in the setup program.
Syntax
SERIESSUM(x,n,m,coefficients)
X is the input value to the power series.
N is the initial power to which you want to raise x.
M is the step by which to increase n for each term in the series.
Coefficients is a set of coefficients by which each successive power of x is multiplied. The number of values in coefficients determines the number of terms in the power series. For example, if there are three values in coefficients, then there will be three terms in the power series.
Remark
If any argument is nonnumeric, SERIESSUM returns the #VALUE! error value.
Example
The example may be easier to understand if you copy it to a blank worksheet.
How to copy an example
 Create a blank workbook or worksheet.
 Select the example in the Help topic.
Note Do not select the row or column headers.
Selecting an example from Help
 Press CTRL+C.
 In the worksheet, select cell A1, and press CTRL+V.
 To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.

A 
Coefficients 
=PI()/4 
1 
=1/FACT(2) 
=1/FACT(4) 
=1/FACT(6) 
Formula 
Description (Result) 
=SERIESSUM(A2,0,2,A3:A6) 
Approximation to the cosine of Pi/4 radians, or 45 degrees (0.707103) 
