Ta có: $∆MNP \sim ∆DEF$
$\Rightarrow \dfrac{MN}{DE} = \dfrac{NP}{EF} = \dfrac{MP}{DF} = k$
Do $NP > MP > MN$
nên $EF> DF > DE$
$\Rightarrow EF = 8,5 \,cm$
$\Rightarrow k = \dfrac{10}{8,5} = \dfrac{20}{17}$
$\Rightarrow \begin{cases}DF = \dfrac{MP}{k} = \dfrac{8}{\dfrac{20}{17}} = \dfrac{34}{5} \, cm\\DE = \dfrac{MN}{k} = \dfrac{5}{\dfrac{20}{17}} = \dfrac{17}{4} \, cm\end{cases}$