Question

An auditorium has 21 rows of seats.

The first row has 18 seats and each suceeding row has two more seats than the previous row.

how many seats are in the auditorium?

Answer 1

use sum of arithmetic series

Answer 2

\[sn=(n[2a+(n-1)d])/2\]

Answer 3

This is the sum of an arithmetic series and needs the formula:
\[S=\frac{n}{2}(2a+(n-1)d)\]
n = 21
a = 18
d = 2
Just plug these values into the formula.

Answer 4

let
n=total no of rows
a=number of seats in first row
and d=is the difference of seats between two consecutive rows
so n=21
a=18
and d=2