Question

If an = n(n+2), what is a9?


20


99


108


120

Answer 1

$a_{n} = n(n + 2) \\ a_{9} = 9(9 + 2) = 99$

Answer 2

 

an = n(n+2)

⇒  a9 = 9(9+2) = 9 × 11 = 99