[tex]\it \left.\begin{aligned} \hat E+\hat F=90^o\\ \\ \hat E=2\hat F \end{aligned} \right\} \Rightarrow 2\hat F+\hat F=90^o \Rightarrow 3\hat F=90^o|_{:3} \Rightarrow \hat F=30^o[/tex]
[tex]\it Teorema\ unghiului\ de\ 30^o \Rightarrow\begin{cases}\it DE=\dfrac{EF}{2}\\ sau\\ \it EF=2DE\end{cases}[/tex]
[tex]\it a)\ DE=\dfrac{EF}{2}\Rightarrow DE=\dfrac{27}{2}=13,5\ cm\\ \\ \\ b)\ EF=2\cdot DE \Rightarrow EF=2\cdot9,8=19.6\ cm[/tex]
