Đáp án:
$\begin{array}{l}
\left\{ \begin{array}{l}
{u_1} + {u_4} = 27\\
{u_3}.{u_2} = 72
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
{u_1} + {u_1}.{q^3} = 27\\
{u_1}{q^2}.{u_1}q = 72
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
{u_1}\left( {1 + {q^3}} \right) = 27\\
u_1^2.{q^3} = 72
\end{array} \right.\\
\Rightarrow \frac{{{q^3}}}{{{{\left( {1 + {q^3}} \right)}^2}}} = \frac{{72}}{{{{27}^2}}} = \frac{8}{{81}}\\
\Rightarrow 81{q^3} = 8{\left( {{q^3} + 1} \right)^2}\\
\Rightarrow 8{q^6} - 65{q^3} + 8 = 0\\
\Rightarrow q = 8/q = \frac{1}{8}\\
\Rightarrow {u_1} = \frac{1}{{19}}/{u_1} = \frac{{512}}{{19}}
\end{array}$
