java - Convert loop into mathematical equation

Question

How would I represent this for loop in a mathematical equation?

double n = 1;
    double x = Math.pow(3, n);
    for(double i = 0; i < n; i++){
        x = x + Math.pow(3, i);
    }

Sorry wrote it in Java in a hurry.

Answer 1

for loop can be simplifed to

x= (pow(3,n)-1)/2

I have double checked this here:
http://www.wolframalpha.com/input/?i=sum+pow%283%2Ci%29+for+i+from+0+to+%28n-1%29