The numbers of seats in the first 12 rows of a high-school auditorium form an arithmetic sequence. the first row has 9 seats. the second row has 11 seats. a) write a recursive formula to represent the sequence. b) write an explicit formula to represent the sequence. c) how many seats are in the 12th row?

A₁ = 9
a₂ = 9+2  → a₂ = a₁ + 2. Since it's an AP, d, the common difference is 2

1) Then the recursive formula is :
a(n) = a(n-1) + 2

2) Explicit formula:
It's an AP, with first term a₁, d= common difference and n = number of term, which is the number of rows in this problem.
the value of the nth term is given by the formula is:
value of nth term  = a₁ + (n-1).d

3) Number of seats in the 12th row:

9 + (12-1).2 = 31 seats