You need to use the formula SERIESSUM.

FInd below an extract of the help file

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, run the Setup program to install the Analysis ToolPak. After you install the Analysis ToolPak, you must enable it by using the Add-Ins command on the Tools menu.

How?

Syntax

SERIESSUM(x,n,m,coefficien

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.

Hope this helps

Frazer