Chapter 11 - Section 11.1 - Sequences - Exercise Set - Page 639: 42

$a_{n}=3n+9$ No. of seats in 8th row $=33$

No. of seats in first row $a_{1} = 12 =3+9 = 3 \times 1 +9$ No. of seats in second row $a_{2} = 15 =6+9 = 3 \times 2 +9$ No. of seats in third row $a_{3} = 18 =9+9 = 3 \times 3 +9$ No. of seats in nth row $a_{n} = 3 \times n+9$ No. of seats in 8th row $a_{8} = 3 \times 8 +9$ $a_{8} = 24 +9$ $a_{8} = 33$