9. If the universal set $I=\mathbf{R}, f(x), g(x)$ are both quadratic functions of $x$, $P=\{x \mid f(x)<0\}, Q=\{x \mid g(x) \geqslant 0\}$, then the solution set of the system of inequalities $\left\{\begin{array}{l}f(x)<0 \\ g(x)<0\end{array}\right.$ can be expressed using $P, Q$ as $\qquad$.
9. Analysis Since the solution set of $g(x) \geqslant 0$ is $Q$, the solution set of $g(x)<0$ is $\complement_{1} Q$, therefore the solution set of $\left\{\begin{array}{l}f(x)<0 \\ g(x)<0\end{array}\right.$ is $P \cap \complement_{1} Q$.

Comment This problem uses inequalities as a carrier, focusing on the concepts and operations of the complement and intersection of sets, it is flexible but not difficult.