The Excel SERIESSUM function returns the sum of a power series, based on the following power series expansion:
The syntax of the function is :
Where the function arguments are:
x    The input value to the power series 
n    The initial power, to which x is to be raised 
m    The step size that n is increased by, on each successive power of x 
coefficients    An array of coefficients that multiply each successive power of x 
The number of values in the supplied coefficients array defines the number of terms in the power series. This is illustrated in the following example.
In the example below, the Excel Seriessum function is used to calculate the power series:
The formula is shown in the spreadsheet on the left and the result is shown in the spreadsheet on the right.
Formulas:

Results:

In the example below, the Excel Seriessum function calculates the power series:
Again, the formula is shown in the spreadsheet on the left and the result is shown in the spreadsheet on the right.
Formulas:

Results:

More details and examples of the Excel Seriessum function are provided on the Microsoft Office website.
If you get an error from the Excel Seriessum Function, this is likely to be one of the following:
#VALUE!    Occurs if any of the supplied arguments are nonnumeric 
#NAME?   
Occurs when Analysis ToolPak addin is not enabled in your Excel. + Show How to Do This in Excel 2003: 