Giải thích các bước giải:
Xét $\Delta AND,\Delta AMB$ có:
$\hat D=\hat B$
$\widehat{AND}=\widehat{AMB}(=90^o)$
$\to\Delta AND\sim\Delta AMB(g.g)$
$\to \dfrac{AN}{AM}=\dfrac{AD}{AB}$
$\to \dfrac{AN}{AM}=\dfrac{AB}{BC}$
$\to \dfrac{AN}{AB}=\dfrac{AM}{BC}$
Mà $\widehat{NAM}=360^o-\widehat{ANC}-\widehat{AMC}-\widehat{NCM}=180^o-\hat C=\widehat{ABC}$
$\to \Delta AMN\sim\Delta BAC(c.g.c)$