[tex]ED\perp(ABC)\ \ \ \ (1)[/tex]
[tex]DA\perp AB\ \ \ \ (2)[/tex]
[tex]DA, AB \subset (ABC)\ \ \ \ (3)[/tex]
[tex](1), (2), (3) \stackrel{T.3 \perp}{\Longrightarrow} EA\perp AB \Longrightarrow d(E, AB)=EA[/tex]
Aplicam T. Pitagora in triunghiul EDA si aflam EA=5 cm