Đáp án: D; B
Giải thích các bước giải:
$\begin{array}{l}
4)\Delta A'B'C' \sim \Delta ABC\\
\Rightarrow \dfrac{{A'B'}}{{AB}} = \dfrac{{A'C'}}{{AC}} = \dfrac{{B'C'}}{{BC}}\\
\Rightarrow \dfrac{{A'B'}}{8} = \dfrac{{A'C'}}{6} = \dfrac{{25}}{{10}} = \dfrac{5}{2}\\
\Rightarrow \left\{ \begin{array}{l}
A'B' = \dfrac{5}{2}.8 = 20\\
A'C' = \dfrac{5}{2}.6 = 15
\end{array} \right.\\
\Rightarrow D\\
5)\\
\Delta ABC \sim \Delta DEF\\
\Rightarrow \dfrac{{{C_{ABC}}}}{{{C_{DEF}}}} = k = \dfrac{3}{5}\\
\Rightarrow {C_{DEF}} = 12:\dfrac{3}{5} = 20\left( {cm} \right)\\
\Rightarrow B
\end{array}$
