Answer
The domain $D$ consists of all points $\left( {I,R} \right)$ such that
$D = \left\{ {\left( {I,R} \right)|I\cdot R \ge 0} \right\}$
Work Step by Step
The function $F\left( {I,R} \right) = \sqrt {IR} $ is defined only when both $I,R \ge 0$ or both $I,R \le 0$. Thus, the domain $D$ consists of all points $\left( {I,R} \right)$ such that
$D = \left\{ {\left( {I,R} \right)|I\cdot R \ge 0} \right\}$
