Make UDF for Mathematical Sum function

Hi,

I am very new to Excel. I want to make a sum function that acts like the one I'm familiar with from Math class. I specifically want to make a custom function that allows me to make this sum

Sum of x^i, as i goes from y to z (in increments of one)

so if x = .9, y=3, and z = 5

.9^3 + .9^4 + .9^5

(which is equal to 1.97559)

Perhaps I don't need a custom function for this but I couldn't figure out a way to do this with the regular Excel functions. (If someone can make a "general" sum function, that would be even cooler, but not necessary. I need only one that sums x^i) Thanks!

Thanks so much! That will work.

I also realized right after that I could just write it as the sum of a geometric series,

=((x^y-x^(z+1))/(1-x)

There's more than one way to skin a cat!