[tex]\it \dfrac{AD}{DB} = \dfrac{2}{3} \stackrel{derivare}{\Longrightarrow} \dfrac{AD}{DB+AD} = \dfrac{2}{3+2} \Rightarrow \dfrac{AD}{AB} = \dfrac{2}{5}\ \ \ \ (*)
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DE||BC \stackrel{T.f.a.}{\Longrightarrow} \Delta ADE \sim \Delta ABC \Rightarrow \dfrac{AD}{AB} = \dfrac{DE}{BC} \stackrel{(*)}{\Longrightarrow} \dfrac{2}{5} = \dfrac{DE}{25} \Rightarrow
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\Rightarrow DE = 10\ cm[/tex]