[tex]\it \Delta ABC \sim \Delta DE F \Rightarrow \dfrac{AB}{DE} = \dfrac{BC}{EF}= \dfrac{AC}{DF} \Rightarrow \dfrac{7}{DE} = \dfrac{8}{EF}= \dfrac{10}{DF} =
\\\;\\ \\\;\\
= \dfrac{7+8+10}{DE+EF+DF} =\dfrac{25}{75} =\dfrac{1}{3} \Longrightarrow \begin{cases}\it DE=3\cdot7=21\ cm
\\ \\
\it EF = 3\cdot8=24\ cm
\\ \\
\it DF = 3\cdot10=30\ cm\end{cases}[/tex]
