1. The problem statement, all variables and given/known data Hello there! I'm trying to find the sum of: 1 - 3 + 3^2 - 3^3 + ... + 3^(2n) = ... 3. The attempt at a solution Well, I tried with 1 - x + x^2 - x^3 + ... = (1 - x^n)/(1 - x) with x = 3. However, the nth term doesn't fit (3^(2n)). At the moment, I'm a little bit out of ideas. Any suggestion would be very appreciated.