Giải thích các bước giải:
Xét $\Delta ANE,\Delta AMD$ có:
Chung $\hat A$
$\widehat{ANE}=\widehat{AMD}=90^o$
$\to\Delta ANE\sim\Delta AMD(g.g)$
$\to \dfrac{AN}{AM}=\dfrac{AE}{AD}$
Xét $\Delta AEC, \Delta ADB$ có:
Chung $\hat A$
$\widehat{AEC}=\widehat{ADB}=90^o$
$\to\Delta AEC\sim\Delta ADB(g.g)$
$\to \dfrac{AE}{AD}=\dfrac{AC}{AB}$
$\to \dfrac{AN}{AM}=\dfrac{AC}{AB}(=\dfrac{AE}{AD})$
$\to \dfrac{AM}{AB}=\dfrac{AN}{AC}$
$\to MN//BC$