Giải thích các bước giải:

a.Vì $AM=\dfrac13AB$

$\to \dfrac{AM}{AB}=\dfrac13$

$\to \dfrac{AM}{AB-AM}=\dfrac1{3-1}$

$\to \dfrac{AM}{MB}=\dfrac12$

$\to \dfrac{AM}{MB}=\dfrac{AN}{NC}$

$\to MN//BC$

b.Vì $MK//AC$

$\to \dfrac{MK}{AC}=\dfrac{BK}{BC}$

$\to MK=\dfrac{BK.AC}{BC}$