[tex]\it a)\ AD=\sqrt{4\cdot16}=2\cdot4=8\ cm\\ \\ b)\ BD=\dfrac{AD^2}{DC}=\dfrac{36}{9}=4\ cm\\ \\ d)\ BD=x \Rightarrow DC=9x\\ \\ BD+DC=BC \Rightarrow x+9x=20 \Rightarrow 10x=20 \Rightarrow x=2 \Rightarrow \begin{cases} \it BD=2\ cm,\\ \\ \it DC=9\cdot2=18\ cm \end{cases}\\ \\ \\ AD=\sqrt{BD\cdot DC}=\sqrt{2\cdot9}=3\sqrt2\ cm[/tex]