Đáp án:
Áp dụng định lý Ta lét ta có:
$\begin{array}{l}
+ MN//BC\\
\Rightarrow \dfrac{{AM}}{{AB}} = \dfrac{{AN}}{{AC}} = \dfrac{1}{3}\\
\Rightarrow \dfrac{{CN}}{{CA}} = \dfrac{2}{3}\\
+ NP//AB\\
\Rightarrow \dfrac{{CN}}{{CA}} = \dfrac{{CP}}{{CB}} = \dfrac{2}{3}\\
\Rightarrow \dfrac{6}{{CB}} = \dfrac{2}{3}\\
\Rightarrow CB = 9\left( {cm} \right)
\end{array}$
Vậy BC=9cm