Illustrative Example 2. Suppose in illustrative example 1 you are asked to find the total number of seats, how will you do it? Instead of adding the number of seats per row, you must use the formula. Given in the problem : a1 = 20 number of seats in the first row n = 30 rows d = 2 common difference Solve: 30 5. = - 022, + (1 - 1)d S, (2020) + (30 - 1)2) 5, = 15 (40+29(2)) = 1470 total number of seats ​