1. Let's write an arithmetic sequence in general terms.

2. So this is one way to define an arithmetic sequence.

3. So the arithmetic series is just the sum of an arithmetic sequence.

4. So this is an immediate giveaway that this is not an arithmetic sequence.

5. Now what I want to do is think about what the sum of this arithmetic sequence would be.

6. I said well, it looks like the sum of an arithmetic sequence could be written as perhaps the average of the first term a1 plus an.