Sum of a * b^t from t=1 to 321

[eriagg]

I am trying to Sum the total from the following equation B(t)=a * b^t

So I have $$\sum$$ a * b^t with t=1 to 321

Trying to solve for an equation and getting a(b^(t+1) - 1) / (b-1)

Answer is not correct...help

 

[mathman]

The answer you are presenting is for t starting at 0 not 1. The correct result would be:
ab(b³²¹ - 1)/(b - 1).