答案：B
設 $S(n) = n^2 + 2n$。則該數列的第 5 項
設 $S(n) = n^2 + 2n$。則該數列的第 5 項
$\begin{array}{cl}
= & S(5) – S(4) \\
= & [(5)^2 + 2(5)] – [(4)^2 + 2(4)] \\
= & 11
\end{array}$
