2009-II-42

Ans: B
Let

S (n) = n^{2} + 2 n

. Then the 5th term of the sequence

$\begin{array}{cl} = & S (5) - S (4) \\ = & [(5)^{2} + 2 (5)] - [(4)^{2} + 2 (4)] \\ = & 11 \end{array}$

2009, HKCEE, Paper 2 Sequences