Find the 15th term of the A.P. with second term 11 and common difference 9.

We know that,

⇒ a_n = a + (n - 1)d

⇒ a₂ = a + (2 - 1)d

⇒ 11 = a + d

⇒ 11 = a + 9

⇒ a = 11 - 9 = 2.

a₁₅ = a + (15 - 1)d

= 2 + 14 × 9

= 2 + 126

= 128.

Hence, 15th term = 128.