Bạn viết sai đề rồi . Sửa đề lại nhé ....
a )
Ta có : \(\left\{{}\begin{matrix}\dfrac{3,2}{4,8}=\dfrac{2}{3}\\\dfrac{2,4}{3,6}=\dfrac{2}{3}\end{matrix}\right.\) \(\Rightarrow\) \(\dfrac{3,2}{4,8}=\dfrac{2,4}{3,6}\) \(\Rightarrow\) \(\dfrac{AD}{AB}=\dfrac{AE}{AC}\)
Xét \(\Delta ADE\) và \(\Delta ABC\) ta có :
\(\left\{{}\begin{matrix}\widehat{A}:chung\\\dfrac{AD}{AB}=\dfrac{AE}{AC}\left(cmt\right)\end{matrix}\right.\)
\(\Rightarrow\Delta ADE\sim\Delta ABC\left(c-g-c\right)\)
b )
\(\Delta ADE\sim\Delta ABC\) ( câu a )
\(\Rightarrow\) \(\dfrac{AD}{AB}=\dfrac{DE}{BC}\) \(\Rightarrow\) \(DE=\dfrac{AD.BC}{AB}=\dfrac{3,2.6,4}{4,8}=4,27\)
Vậy \(DE=4,27cm\)
