Summation in Excel

I'm not certain I fully understand the summation you want (you mention n, but I don't see an n in the function, and so on).

Basic strategy I almost always use for a summation:

1) Put any constants into cells
2) Create a column for the counter variable (X in this case??)
3) Create a column to calculate each addend for each value of the counter variable
4) Use a =SUM() function a the top or bottom of this column to sum up the addends.

So it will look something like:

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