9 18 36
9 * 2 18 * 2
so notice, to get the next term, we simply multiply the "current" one by 2, namely is a geometric sequence, with a "common ratio" of 2, and we know the first term's value is 9.
[tex]\bf n^{th}\textit{ term of a geometric sequence}\\\\
a_n=a_1\cdot r^{n-1}\qquad
\begin{cases}
n=n^{th}\ term\\
a_1=\textit{first term's value}\\
r=\textit{common ratio}\\
----------\\
a_1=9\\
r=2
\end{cases}\implies a_n=9\cdot 2^{n-1}[/tex]
