[tex]\it \dfrac{BD}{DC} = \dfrac{4}{9} \Rightarrow \dfrac{BD}{4} = \dfrac{DC}{9} =\dfrac{BD+DC}{4+9} = \dfrac{BC}{13} = \dfrac{26}{13} = 2\Rightarrow
\\ \\ \\
\Rightarrow \begin{cases}\it BD=4\cdot2=8cm\\ \\
\it DC = 9\cdot2= 18cm\end{cases}[/tex]
[tex]\it T.h. \Rightarrow AD^2= BD\cdot DC=8\cdot18=144\Rightarrow AD=\sqrt{144} = 12cm[/tex]