3. Given $\triangle O A B$ where $\angle A O B<90^{\circ}$, from any point $M(\neq 0)$ in $\triangle O A B$, draw perpendiculars $M P$ and $M Q$ to $O A$ and $O B$ respectively. Let $H$ be the orthocenter of $\triangle O P Q$. What is the locus of point $H$ as point $M$ traverses segment $A B$?
(IMO - 7 problem)
3. Let the projections of $P, A$ on the line $O B$ be $K, D$, and the projections of $Q, B$ on the line $O A$ be $T, C$. The desired locus is the line segment $C D$.

