11. As shown in Figure 5, in 
$\square A B C D$, $A M \perp$ $B C, A N \perp C D, M, N$ are the feet of the perpendiculars. If $A B=13$, $B M=5, M C=9$, then the length of $M N$ is $\qquad$
$11.13 \frac{11}{13}$.
As shown in Figure 5, connect $M N$, and draw $M H \perp A N$, with the foot of the perpendicular at $H$. We can obtain
$$
\begin{array}{l}
\angle B=\angle D=\angle M A N, \\
\triangle A B M \backsim \triangle M A H \backsim \triangle A D N .
\end{array}
$$