MN // BC theo định lý Ta-let ta có:
$\dfrac{AM}{AB}$ = $\dfrac{AN}{AC}$ = $\dfrac{MN}{BC}$
+)$\dfrac{AM}{AB}$ = $\dfrac{AN}{AC}$ `<=>` AB = $\dfrac{AM.AC}{AN}$ = $\dfrac{4.8,5}{5}$ = 6,8cm
`<=>` MB = AB - AM = 6,8 - 4 = 2,8 cm
`<=>` $\dfrac{MB}{AB}$ = $\dfrac{2,8}{6,8}$ = $\dfrac{7}{17}$
+) $\dfrac{MN}{BC}$ = $\dfrac{AN}{AC}$ = $\dfrac{5}{8,5}$ = $\dfrac{10}{17}$
+) MB = 2,8 cm
