3. In trapezoid $A B C D$, $A B / / C D, A C 、 B D$ intersect at point $O$. If $A C=5, B D=12$, the midline length is $\frac{13}{2}$, the area of $\triangle A O B$ is $S_{1}$, and the area of $\triangle C O D$ is $S_{2}$, then $\sqrt{S_{1}}+\sqrt{S_{2}}$ $=$ . $\qquad$
3. $\sqrt{30}$