a, Xét ΔANM và ΔABC có :
\(\dfrac{AN}{AB}=\dfrac{3}{6}=\dfrac{1}{2}\)
\(\dfrac{AM}{AC}=\dfrac{3,5}{7}=\dfrac{1}{2}\)
\(\Rightarrow\dfrac{AN}{AB}=\dfrac{AM}{AC}\left(=\dfrac{1}{2}\right)\)
Mà \(\widehat{A}:chung\)
\(\Rightarrow\Delta ANM\sim\Delta ABC\left(c-g-c\right)\)
b, Ta có : \(\Delta ANM\sim\Delta ABC\left(cmt\right)\)
\(\Rightarrow\dfrac{AN}{AB}=\dfrac{NM}{BC}\)
hay \(\dfrac{3}{6}=\dfrac{MN}{8}\)
\(\Rightarrow MN=\dfrac{3.8}{6}=4\left(cm\right)\)
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