Question

An auditorium has 20 seats on the first row, 24 seats on the second row, 28 seats on the thirdrow, and so on and has 30 rows of seats. How many seats are in the theatre?

Answer 1

The number of seats forms an arithmetic sequence

The number of seats increasing by 4

There are 30 rows

We need to find the total number of seats

We will use the rule of the sum of the arithmetic sequence

$S_n=\frac{n}{2}(2a+\lbrack n-1\rbrack d)_{}$