Giải thích các bước giải:

a.Vì $DE//AM$

$\to \widehat{BED}=\widehat{BAM},\widehat{BDE}=\widehat{BMA}$

$\to \Delta BDE\sim\Delta BMA(g.g)$

b.Ta có: $DE//AM$

               $M$ là trung điểm $BC\to MB=MC$

$\to \dfrac{DF}{AM}=\dfrac{CD}{CM}$

c.Ta có: $DF//AM$

$\to \dfrac{DE}{AM}+\dfrac{DF}{AM}=\dfrac{BD}{MB}+\dfrac{CD}{CM}=\dfrac{2BD}{2MB}+\dfrac{2CD}{2CM}=\dfrac{2BD}{BC}+\dfrac{2CD}{BC}=\dfrac{2BD+2CD}{BC}=\dfrac{2BC}{BC}=2$