18. In the figure below, $A B$ and $C D$ are parallel chords of a circle with centre $O$ and radius $r \mathrm{~cm}$. It is given that $A B=46 \mathrm{~cm}, C D=18 \mathrm{~cm}$ and $\angle A O B=3 \times \angle C O D$. Find the value of $r$.