Solution:
(i) the ninth term of the G.P. $1,4,16,64, \ldots$
It is known that,
$t_{1}=a=1, r=t_{2} / t_{1}=4 / 1=4$
Using the formula.
$\begin{array}{l}
\mathrm{T}_{\mathrm{n}}=\mathrm{ar}^{\mathrm{n}-1} \\
\mathrm{~T}_{9}=1(4)^{9-1} \\
=1(4)^{8} \\
=4^{8}
\end{array}$

(ii) the $10^{\text {th }}$ term of the G.P. $-3 / 4,1 / 2,-1 / 3,2 / 9, \ldots$
It is known that,
$t_{1}=a=-3 / 4, r=t_{2} / t_{1}=(1 / 2) /(-3 / 4)=1 / 2 \times-4 / 3=-2 / 3$
Using the formula,
$\begin{array}{l}
T_{n}=a r^{n-1} \\
T_{10}=-3 / 4(-2 / 3)^{10-1} \\
=-3 / 4(-2 / 3)^{9} \\
=1 / 2(2 / 3)^{8}
\end{array}$