Question 18
Let $A O B$ be a $60$-degree angle. For any point $P$ in the interior of $\angle A O B$, let $A'$ and $B'$ be the feet of the perpendiculars from $P$ to $A O$ and $B O$ respectively. Denote by $r$ and $s$ the distances $O P$ and $A' B'$. Find all possible pairs of real numbers $(r, s)$.