Hi, and welcome to the forum.

I don't know of an inbuilt Sigma type function in standard Excel, but no doubt a user defined function (UDF) in Visual Basic could be written. Before doing that though does the following worksheet solution help.

In A2 enter your x constant, in B2 your a constant and in C2 =Max(A5:A104)

In A5 to A14 enter your n numbers starting at 1 and presumably incrementing by 1, ending in 10. Now in B5 and copied down enter
=(((1-$A$2)^A5-1)*$A$2)*$B$2
and finally in B4 enter =Sum(B5:B104)

If that gives you what you're looking for and you prefer a UDF solution, then post back.

Regards