Ans: C
Let $a$ and $r$ be the first term and the common ratio of the geometric sequence. Hence, we have
Let $a$ and $r$ be the first term and the common ratio of the geometric sequence. Hence, we have
$\left\{ \begin{array}{ll}
a \times ar = 18 & \ldots \unicode{x2460} \\
ar^2 \times ar^3 =288 & \ldots \unicode{x2461}
\end{array} \right.$
$\unicode{x2461} \div \unicode{x2460}$, we have
$\begin{array}{rcl}
r^4 & = & 16 \\
r & = & 2 \ \text{ or } -2\text{ (rejected)}
\end{array}$
Sub. $r=2$ into $\unicode{x2460}$, we have
$\begin{array}{rcl}
a^2 (2) & = & 18 \\
a^2 & = & 9
\end{array}$
Therefore, the product of the 4th term and the 5th term of the sequence
$\begin{array}{cl}
= & ar^3 \times ar^4 \\
= & a^2 r^7 \\
= & 9 \times 2^7 \\
= & 1152
\end{array}$
