Đáp án:
$\begin{array}{l}
1)\dfrac{{AD}}{{AB}} = \dfrac{2}{5}\\
2)\\
Trong:\Delta ABC:DE//BC\\
Theo\,Talet:\\
\dfrac{{AD}}{{AB}} = \dfrac{{AE}}{{AC}} = \dfrac{{DE}}{{BC}} = \dfrac{2}{5}\\
Vậy\,\dfrac{{DE}}{{BC}} = \dfrac{2}{5}\\
3)\\
\dfrac{{DE}}{{BC}} = \dfrac{2}{5}\\
\Rightarrow DE = \dfrac{2}{5}.BC = \dfrac{2}{5}.10 = 4\left( {cm} \right)\\
Vậy\,DE = 4cm
\end{array}$