Given right triangle ABCABC with altitude \overline{BD}BD drawn to hypotenuse \overline{AC}AC. If AB=2AB=2 and AC=4,AC=4

Given right triangle ABC with altitude$\overline{BD}$ drawn to hypotenuse $\overline{AC}$.
If $\overline{AB}=2$ and $\overline{AC}=4$,
what is the length of $\overline{AD}$?

$\begin{array}{|rcll|} \hline \overline{AB}^2 &=& \overline{AD}\times \overline{AC} \\ 2^2 &=&\overline{AD}\times 4 \\ 4 &=&\overline{AD}\times 4 \\ \mathbf{\overline{AD}} &=& \mathbf{1} \\ \hline \end{array}$