a/ $\dfrac{AM}{AB}=\dfrac{2,5}{10}=\dfrac{1}{4}$
$\dfrac{AN}{AC}=\dfrac{3}{12}=\dfrac{1}{4}$
$→\dfrac{AM}{AB}=\dfrac{AN}{AC}(=\dfrac{1}{4})$
$→MN//BC$ (Định lý Talet đảo)
b/ $MN//BC$
$→\dfrac{AM}{AB}=\dfrac{MN}{BC}$ (Định lý Talet)
$→\dfrac{1}{4}=\dfrac{MN}{15}$
$→\dfrac{1}{4}.15=MN$
$→3,75=MN$