How to find the sum of a series?

Right then. Finding the sum of a series entirely depends on the type of series you have. Your first job is to identify it.

Check if there’s a common difference between the terms. If so, it’s an arithmetic series. The sum is just the number of terms times the average of the first and last term. Simple as that.

If there's a common ratio instead, you've got a geometric series. There's a specific formula for this which is easy enough to find. If your series is neither, things get a bit more advanced, but these two will cover most cases you'll see. Cheers.