Summing numbers which increase by a fixed amount (arithmetic progression) An auditorium has 21 rows of seats. The first row has 18 seats, and each succeeding row has two more seats than the previous row. How many seats are there in the auditorium?
Now I supposed you could use sigma notation since this kind of problem reminds me of it, but I have little experience using it so I'm not sure.

$$s_1 = 18 = 18 + 2(1-1)\\
s_2 = 20 = 18 + 2(2-1) \\
s_3 = 22 = 18 + 2(3-1) \\
\ldots \\
s_{i} = 18 + 2(i - 1) \\
\ldots \\
s_{21} = 18 + 2(21 - 1)$$
Sum:
$$\sum_{i=1}^{21} (18 + 2(i-1))$$
